The title is odd since the calculations you have shown are indeed correct, but it looks like you are wondering why it is necessary to append UL to unsigned long numbers.
The #define statement just does a textual substitution so as long as your 1250000 lands somewhere where an unsigned long is handled correctly, it will work.
But try this as your first line :
#define maxStpT 125 * 10000
and then this:
#define maxStpT 125UL * 10000
and you should notice a difference. The preprocessor treats such calculations as int unless somehow forced to do otherwise.