What agreat forum - have only just joined!

I have recenly st up my own darkroom and have been doing quite a bit of enlarging.

One question I have is:

Say I decide that 30 seconds exposure is about right for my 5X5 print I am about to make.

Is there any calculation to enable me to decide on comparable exposure if I increase the size of the enlargement to say 8X8?

I have discovered that as the enlargement is bigger I need to expose for longer, but sometimes I don't want to have to go through the busines of making test strips.

Mabey there is a rough estimation or an actual calculation?

Thanks for any help,