Either my calculator doesn't work or I should return university diploma:
((New elevation / Old Elevation) ^2) X Original Exposure
Supposing your 8x10 elevation from lens to paper was 24 inches @ 16 seconds and your New elevation is 30
inches, then: ((30/24)^2) = (1,25)^2)=1,5625
1,56 X Original Exposure (16 seconds) = 25s New Elevated Exposure.