Chain serves a few purposes. First, it provides a connection from the anchor
to the bow of the boat so it must be strong enough. Second, it provides chafe protection for your anchor rode
so how much chain you need is determined by whether you anchor
in a place with chafe. Third, chain will help your anchor
set because it will keep the shank on the sea floor.
What are the absolute worst conditions that you expect to anchor in? Also, what type of boat do you have? The chain needs to be strong enough for this situation. For reference, the ABYC recommends a working load of 1500 lbs for your boat on a permanent mooring. 1/4" G40 has a working load of 2600 lbs so that would be more than adequate. Just make sure to replace your chain once it starts to significantly deteriorate.
Since you have already settled on a length, I am going to assume that you have taken into account whether you will be dealing with coral or anything else that will require the use of more chain.
What model of anchor do you have? For most anchors, setting can be helped by more, heavier chain. The exception to this that I know of is fortress
anchors in very soft bottoms but they have good guidelines on their website. If you plan to anchor on short scope, heavier chain will be necessary but if you plan to use adequate scope, then the 1/4" chain mentioned above will be fine. For short scope anchoring, I would consider 5/16" G30.