so you have plans to change your tuning at times? for what purpose? the only reason you should need to do that is for competition. only other reason is if you are trying to figure out for example how 30hz vs 40hz differs in sound with your sub. you would have to have a well trained ear to notice a difference between a few hz in tuning...
slot port may not be as hard as you think. all you need are box dimensions, and net volume. there's plenty of smart folks on here that can tell you exactly what dimensions you need after that. slot ported enclosures are also more structurally rigid, but its your box and your call. 35hz seems like a good number. i would use anywhere from there to 38hz.