I don't think you can make a 16x20 with the Printmaker 35, baseboard or otherwise. The bellows won't compress enough. You're better off trying with the Omega. As a general rule, in addition to increasing the exposure time as you go bigger, you'll have to increase the contrast filtration - if it printed well at grade 3 as an 8x10, you'll probably need to use grade 4 (or even higher - maybe a 4 1/2) filtration to get a good 16x20 out of it. If you start printing at variable sizes, I would highly recommend getting something like an Ilford EM-10 enlarging meter. They're quite simple to use - you first get your enlarger set up, and make a good print from a known negative. With the lens aperture and negative-to-paper distance set, put the EM-10's sensor under a (preferably) middle-gray area of the negative on the baseboard. Turn the calibration dial until only the center green light is lit. Note the number so if you have to re-set it later you can. Then you are all set to print the same negative at a different size and/or a different negative at the same size. With the same negative, just re-position the enlarger head at the appropriate height, re-focus, and make sure the probe is in the same area of the negative that you measured. Turn the lens aperture until the single green light is illuminated on the EM-10. With a new negative, put the EM-10's probe in an image area that should have the same tonal value as the area you metered on the original negative. Then adjust the enlarger lens aperture until you have only a single green light. In both cases you don't have to adjust exposure time - just the lens aperture. It's somewhat primitive because it may be better to adjust time than aperture, especially if you are dealing with a very dense negative, and you don't want to print at wide-open. Of course you can calculate the aperture/time changes yourself after you set the exposure based on the meter.
i 2nd the em10 recommendation