Working out a formula for problem

I need help with a spreadsheet i need to work out a formula for my spreadsheet which I use to work out cutting lists for timber frames.

I need it to work out if the width of a job is for eg 2400mm i need to work out how many timber studs I need so the space between each stud is between 400mm and 500mm and this will need to work for a range of different sizes of frames.

I have it written at the moment and it just devides the width by 400 and gives me a amount of studs but it would work much better if it could space them between 400 & 500.

Thanks Pricey