I think I post these pictures every year around this time. That is a Capri 25, with a hinged mast step. Its a deck stepped mast, and the mast is 31 feet off the deck. The mast in question is a masthead rig, with 2 lowers and an upper. The Capri 25 is a fixed keel boat with a spade rudder (no pintles). To accomplish this, the forward lowers ONLY and forestay have been removed.
The critical component isn't the A-frame (which is pretty critical) its the support at the stern, you need to get the mast above horizontal to start, and the higher you can get it to start, the easier the whole process is to implement. I used the spinnaker pole attachment point (on this boat it is on a track and can be raised to nearly eye level)... a Higher attachment point for the mainsheet purchase is better (more leverage). You'll notice I have the mainsheet (4:1) with the cam cleat at the apex of the A-frame. you stand below the Apex of the A-frame, and heave. I faced port, and used my right hand to haul line, and my left is available to "steer" the mast. The great part about this setup is you can stop raising/lowering at any point. If you have anyone at all available they can hold the mast in place, while you sort lines... or in my case (alone)... you can lower back down, sort shrouds/lines, and start again. The legs of the A-frame must be held in place... the Apex must sit well forward of the mast, enough room for the full mainsheet system at its shortest length to fit. I usually angled the A-frame slightly forward to accomplish this task, the picture shows is angled slightly aft, which isn't helpful.
This is a better shot of the deal.
Before you think this can only be used on a particular boat... here is an S2 7.9. The S2 is a heavier boat than the Capri 25 (by 1500lbs) and also has a heavier built mast that is roughly the same height. The S2 7.9 is 26 feet (I'll do that conversion for you), and has single lowers and single uppers, with swept spreaders. This means the only stay removed is the forestay for lowering. You'll note that the mast holder to stern is nearly off the boat, that is because this is a transom hung rudder... it sits on the pintles (gudgeons were on the rudder on the S2 7.9 I know weird). Anyway, I created this mast support to be adjustable, for a lower position while trailering, and adding 4 feet for raising/lowering the mast. In this picture its in the raised position.
I presently have a Wavelength 24, which is also a masthead rig, but with a keel stepped mast, and a spade rudder. It is my contention that there are 2 types of "gin poles." One is attached to the mast at a 90 degree, and winched into horizontal position (mast vert).... the other is a pole that is used as a crane. The latter is the one that I'll be likely employing this year. Actually instead of a gin pole, I'm thinking of building another A-frame, but using IT as a crane. This requires that the A-frame be taller than the mid-point of the mast, which is at least 4 feet longer than it's height above deck. So for my 30 foot deck height mast, that means I have to be at least 17 feet high... My aim is to make it 20 feet total in height. Best I can tell there is no way to manage (easily) a keel stepped mast solo. I am determined (by having spotters available) to find a way, but for now, I'm resolved to accept help to get the stick vertical.
My fall back (since I am behind for the season) is that we have a rather LARGE tree branch across our launch ramp (like 12 inches in diameter), that is easily 24 feet in the air. Our dockmaster used it successfully to raise the stick on a J80 last year (with the help of at least 4 others). This is of a very old hardwood tree. Of course the downside to it, is you have to be in JUST the right spot for it to be helpful to place the stick. This is not something you'd want to do weekly.
Finally, by the implications of your post above, I gather you want to solely trailer-sail your boat. This is a VERY noble thing to do, but at 25 feet it'd be quite a long process to launch, and retrieve in one day. I got pretty good at my S2 7.9 which was easy to launch actually (retractable keel)... but it still took at good 1.5 hours to launch, raise the stick, bend sails on... that's 3 hours out of your day, and you haven't gotten any sailing time in yet! The last boat I successfully trailer sailed that had any kind of performance to it, that I'd argue was "easy" enough to do each time... was my Capri 22. The mast was ALMOST light enough to raise without an A-frame. The rigging was simple and light, and things like the outboard could stay on the motor mount. On that boat the rudder/tiller was light enough to easily throw on quickly at launch too. With the Wing Keel model and deck stepped fractional rig it was almost easy to launch (especially with a trailer tongue extension). Must heavier/bigger than a 22 gets really tiresome to try to launch solo every time you want to sail. Actually I'd argue an 18 footer would be a much more comfortable size for 2 people and solo launch day sails (also no need for an A-frame or gin pole). A boat like a Precision 18, Catalina Capri 18, Victoria 18, or Compac 19.
The Catalina 22 is popular because it launches easy, has a light rig, sails pretty well, and is nearly simple enough to trailer day sail it. It hits every hot button for a multi-purpose boat.
Just my worthless $0.02.