You need to fine tune the timer, not the OSC.
Here is an example
http://www.picbasic.co.uk/forum/show...3259#post23259
And here is another
http://www.picbasic.co.uk/forum/show...5912#post75912
And this will help with the calcs.
http://www.picbasic.co.uk/forum/cont....-PICMultiCalc