need help copying family moveis from mini dvd to reg size dvd  

Anyone know how to figure how many home movies (each about 3-4 minutes long) can be fitted on a regular DVD from a mini dvd camcorder dvd? I record 30 min on each, but assume that the number of time listed on the dvd should be what I can put on there, but it isn't. If that makes sense.

I have a mini dvd cam corder and I record the movies on that, copy them to the PC and then want to copy them to regular size DVD and give to the family, but I do the math to figure how mnay minutes I have to go on to the dvd minutes and it never comes out right, I get"to much for this dvd". So how do I go about copying the right amount of minutes to the dvd without going over what it will accept?

I have windows xp and a sony mini dvd cam corder. Thanks for your help!