I want to build a new 7 ft. rod for 1 handed casting for back bay use. The blank is a Lamiglas GLB841M. Lighter blanks seem just too soft in the tip for my intended use. What grip length would you recommend? Suggested length from top of reel seat to bottom of butt cap? This is a medium action graphite blank that I'll be using lures up to 1 1/4 oz.

totally up to you Barry - experiment and see what feels right.

I fish mostly freshwater, and build mine accordingly. for lighter use, I like the butt to come about 2" short of my elbow when holding the rod.
for heavy use where i may need to put more leverage on the fish, I like one that is a little longer so I can tuck it under my elbow or between myelbow and side.

Totally depends on the preference of the person using it!! That's what makes it "Custom" There is no roght or wrong, it's whatever feels right!

Mike (Southgate, MI)
If I don't want to, I don't have to and nobody can make me (except my wife) cuz I'm RETIRED!!

First thing I do when building a rod is put the reel I'm going to use on the reel seat and slide it down the blank. I'll then simulate casting, fighting,etc and when I find a length I like mark the blank where the bottom of the reel seat. The distance from the bottom of the reel seat to the butt end of the rod is the length of my rear grip, do the same for the fore grip. I think this step is what really makes a rod custom to it's intended user.

I prefer 225mm (9") from the stem of the reel for a slighly heavier single hander and about 150-175mm (6-7") from the stem for an ultralight. I like these lengths because the butt doesn't get caught up in my clothing or, smack my elbow when casting and jigging soft plastics.

This is only my persoanl preference and that is the beauty of a custom.. you can have whatever suits you.

That is not a light weight rod (not 4lb test line), so for one handed casting and one handed fighting it will be nice to let the butt grip lay against your forearm. Depending on the lenght of your arms I would be around 12 or 13". Trigger reel seat would be nice for grip.

Forgot to mention that this rod will be set up for spinning with a Penn 712Z. 12 lb line

First -- as most have already noted the rod is for you so you can make it any length that you like. Personally on spinning rods that are "back bay use" I find myself trying to get that little extra distance and wind up using my left hand to pull the rod around rather than whipping my right wrist to death. I do a lot of "back bay" casting to striped bass, bluefish, jigging for trout & drifting for flounder. I also have learned to appreciate the extra support that I can get from my forearm. The blank you've got is certainly not a limp noodle - and if you're using 12lb test line and setting your drag to 6lb (or less) you could be in for a longish fight . Another consideration is - will you put this rod in a rod holder on a boat from fising hole to fishing hole? If so you'll appreciate the extra length s a security measure against donating the rod to the bottom Anyway for me that turns out to be a rear grip of between 9 & 10 inches - or about 4 inches from the outside of my elbow joint... I'm "average" at 5'7" hope that helps you some.