Well, it's rather long to do it with 8 and 9 but this works... it's more or less 'if starting is 10 and you spend 0 your total is 0, if starting is 10 and you spend 1 your total is 11, ect'. A pain in the butt to type out but once done it should just be a matter of swapping out the B3 and C3 in the formula with the actual cells and pasting it into the target cell. Also n/a can be replaced with whatever message you wish.
B3 is starting stat
C3 is points spent
It'll pop out an n/a if you put in a points spent total that doesn't work.
[SBLOCK=formula] =IF(B3=10,IF((C3=0),10,IF((C3=1),11,IF((C3=2),12,IF((C3=3),13,IF((C3=4),"invalid",IF((C3=5),14,IF((C3=6),"invalid",IF((C3=7),15,IF((C3=8),"invalid",IF((C3=9),16,IF((C3=10),"invalid",IF((C3=11),"invalid",IF((C3=12),17,IF((C3=13),"invalid",IF((C3=14),"invalid",IF((C3=15),"invalid",IF((C3=16),18,"invalid"))))))))))))))))),(IF(B3=8,IF((C3=0),8,IF((C3=1),9,IF((C3=2),10,IF((C3=3),11,IF((C3=4),12,IF((C3=5),13,IF((C3=6),"invalid",IF((C3=7),14,IF((C3=8),"invalid",IF((C3=9),15,IF((C3=10),"invalid",IF((C3=11),16,IF((C3=12),"invalid",IF((C3=13),"invalid",IF((C3=14),17,IF((C3=15),"invalid",IF((C3=16),"invalid",IF(C3=17,"invalid",IF(C3=18,18,"invalid")))))))))))))))))))))) [/SBLOCK]