Not speaking for him but I think all the dimensions and distances for what youre looking for could be obtained by carefully looking at all the 'ratios' of just about any Bob Perry designed cutter rig. Bob is probably THE master of modern design cutter rigs. Simply mathematically ratio
those dimensions to what you have on your boat ... and then send him a nice royalty charge.
For an added forestay (the old forestay now becomes the headstay) you really need to oppose those forces applied onto the mast via either running backstays (best) or intermediate stays (so-so) .... or you can easily overload other rigging components such as backstay, etc.
FWIW. On a cutter rig (solent rigs are similar), being able to easily remove, tension adjust, etc. a forestay (where the forestay-sail
flys) is a big
plus, especially in those VERY light air conditions where a staysail flown 'under' a headsail will adversely interfere with the total optimum output of the total sail plan AND to more easily tack a headsail without getting it fouled on the damn forestay. I offer that the early Island Packets had an easily 'tensionable' forestay that could be totally slacked (even 'stored') and didnt have to be removed/disconnected for the system to work well. What IP did was to use 'soft' rigging wire - attached at the tack, running up to a sheave that was mounted on the mast, and then ran down the mast to a winch. Good idea but a 'materials' failure as wire
does not like to be run over small diameter sheaves ... most of these systems easily broke the wire. Modern ultra-strong polymer line such as dyneema could easily be used for a forestay on a cutter rig in the same manner of the earl IPs with 'soft' wire forestays.
The 'plus' here with modern super strong polymer line is the ability to precisely tension (on the fly) the forestay so that the headstay
doesnt sag off to leeward when both sails are pulling / flying ... a sagging headstay results in TERRIBLE pointing ability while causing the boat to heel over aggressively and skid off to leeward when attempting to sail closehauled. If you dont have the ability to easily change/adjust tensions in the forestay, the backstay sometimes has to apply severe load to get that headstay to give less or proper 'normal' sag ... and you can easily overload the entire rigging.
The use of intermediate stays if you dont want 'runners', attached to the level of the mast connection (hounds) of the forestay, probably could also benefit by using dyneema - less weight aloft.
What Im implying here is that a cutter rig is a total PITA to sail on a beat because of 2 stays in front of the mast (and only one backstay), each with different and variable loading being applied from their respective sails, stretching and sagging independently of one another .... that an independently and EASILY tension-adjustable forestay would be an outstanding plus while making a cutter rig FLY and point as well as any damn sloop (almost). BTW, in most cases when beating with top & staysail flying, the forestay is the one thats best to 'unload' or slacken ... which automatically tensions the headstay because the tension in the backstay doesnt change. With such a system ANY damn sail could be used as the staysail because the luff hollow - the smooth curve cut into the leading edge at the luff of jibs/genoas/staysails - could easily be 'matched' with the proper stay tension ... so the staysail could be flown close to the design shape that the sail designer intended.
The prime job of the rigging is not to hold the damn mast up; but, rather to provide a stable consistent geometric platform so that the jibs/genoas/staysails have a stable SHAPE.... and rigging wire tension is a vital component of what shape results in that headsail/staysail, etc. Cutter rigs can be cranky, recalcitrant, aggressively heeling and slow if you dont realize this 'interplay' of sails and rig tensions.
Here's an article I wrote some time ago which address how the match the headstay (or forestay) sag in the wire to what was CUT into the leading edge of the sail's luff by the sailmaker to help you understand the concept of sagging forestay/headstay wires: http://www.ftp.tognews.com/GoogleFil...f%20Hollow.pdf
Imagine the 'problem' when a single backstay is being adjusted to remedy different sags on each of those two 'wires' in front of that mast - the PROBLEM with cutter (and solent) rigs.
hope this helps