I see. If you feel more comfortable with 37.5 then do that. It’s still full time nonetheless. Be wary of stating “more or less” than 37.5, as less than 37.5 hours could vary between 0 to 37 hours per week, which could signal that there may be some weeks that you’re not working full time.
Maybe you could ask your employer to state that you work at least 37.5 hours per week? So it means it can go higher but not lower than 37.5.