Neck Tenon Length Recommendation?

[j. pierce]

I'm working on a 35" scale, five string bass, for my brother.

It's a set neck, with an LP style tenon; i.e., the tenon is not the full width of the neck where it joins the body.

A it stands now (it is not joined to the body yet), the tenon is about 1 3/4 deep (I may need to remove a bit of that depth - waiting on my bridge to arrive this week before I finalize how far into the body it's going to be recessed) two inches wide, and just under 4 and a half inches long. (Being a narrow tenon, all of that length is supported by wood on both sides, of course)

That's a fairly large tenon, although not nearly as long as some I have done, or some I have seen. This was as much as I could get out of this piece of maple.

Ignoring tone concerns, and focusing on stability and strength of the join, is there a rule of thumb as to how much material/size I want for a mortise and tenon, assuming a good fitting tight joint that's been well glued?

I'd like to chop a bit of length off this tenon (bring it down to about 3.25"-3.5" in length) to put the end of the tenon behind the end of the fingerboard. That way I attach the top first and do the neck pocket through my top and body core woods, rather than jigger around trying fit the top around the neck tenon and then glue three things at once.

Not having worked with a bass like this, I wonder if what the rule of thumb is here.

I tried searching, but nothing popped up. I'm going to go clawing through "In Progress and Finished Work" and see what others have done, but in the mean time, I thought I'd ask here.