Hi there,
From what you have described I would say we do need some clarification of your current rule as Nick has already said, TDA rule 31 as follows:
31. Raises A raise must be at least the size of the largest previous bet or raise of the current betting round. If a player puts in a raise of 50% or more of the previous bet but less than the minimum raise, he or she must make a full raise. The raise will be exactly the minimum raise allowed (see exception for multiple same-denomination chips Rule 33). In no-limit and pot limit, an all-in wager of less than a full raise does not reopen the betting to a player who has already acted.
In a round of betting the BB (The forced BET) amount is in force irrelevant of how much a player is all-in for, to clarify this I will now give some examples (all are NLHE):
400/800 - Pre-flop BB is all-in for 700, players must call at least 800 or raise to 1,600 to enter the hand
200/400 UTG Pre-flop opens for 350 All-in, other players must still call the BB of 400 or raise a minimum of 400
300/600 Pre-flop a player opens for an all-in of 800, the bet faced now is 800, and players may either call it or raise to a minimum of 1,400 total
300/600 Post-Flop 4 players, 1 checks; 2 bets 1,000; 3 shoves for 1,800 total; 4 folds; 1 calls 1,800 (all options available); 2 can either complete 800 more or fold - he cannot raise as he has already acted and player 3's all-in is not a complete bet, but if for example 4 raised to 2,800 total (min) that would re-open betting to 2.
400/800 post-flop, 3 players; 1 checks; 2 checks; 3 shoves for 700, this can only be called by 1 & 2 or fold as it is not a complete bet.
400/800 post-flop First to act shoves for 600, following players may either call 600 or raise to 1,400 total (min)
From what you have described you are correct with what you already have, players in your situation are permitted to call the all-in wager, raise or fold, there is no requirement to complete to a full-raise even though it is not a full raise itself. Everything Nick has already stated I agree fully with - I am also not aware of any rule out there that would not let the player not call the all-in wager in your situation
Hope this helps,
Stuart