In order to price the floating leg of an IRS I am computing forward rates for future coupons, but I'm not sure whether I have to compute such rates between reset dates or between start dates.

My intuition tells me that forward rates should be calculated between reset dates because that's when you fix the rate for each coupon, but I've seen that in practice many people calculates them between start dates and it confuses me a little bit, because my logic says that in such scenario you'd be estimating a rate that is already known.

I hope you guys can help me.
Thanks!