moordog,
That's a great question. Years ago that was the main game that we played in Las Vegas. It was stud, but it was spread limit. I believe the best way to determine the legal amount, or required amount or %, if you will, is to always consider the minimum amount in the spread limit as the bring. This bring-in is the legal amount that is recognized on every street. Something to consider when playing spread limit: I remember playing $1 to $5 stud and having the betting capped at $4...Player A bets $1, next Player raises to $2, next $3 and capped at $4. So to answer your question, there is nothing to gauge your raise on except the previous bet. i.e. $2 to $10...Player opens for $6 (a full bet) next player wants to raise but only has $9, he goes all-in...now the question is, does the short all-in reopen the raise to the initial bettor? I would say yes, because it is limit poker I would use the 50% rule. I would also count the short all-in raise as one of the raises if there is a 3 or 4 raise limit. Same situation, only the all-in only has $8...because this is less than 50% of the initial $6 bet, it would not reopen the betting to the initial bettor and (of course) not be considered for a limit on raises.
I hope this helps. If you'd like further discussion let me know and I can do some real serious digging.