simple. Keep a cloth tape measure to hand. If old lens to easel distance was 50cm and now it is 70cm, then:

(70 squared) divided by (50 squared) X old exposure time. if old exposure time was 10 seconds, it will now be 19.6

Using the area method is daft as you might adjust the cropping when you change the height. I use the above and it is dead simple. It can be that I reduce the factor ever so slightly at times to account for the bulb getting hotter in longer vs very short exposures, so instead of going from a 5 second (for a very small print) to 25 seconds for a much larger one, I might try 22 or so. I often stop down more for small prints so that I can then keep the time broadly similar by opening up a stop or half more for a larger one. with small prints you are not likely to have an issue with diffraction esp when the neg is big.