I would recommend doing regular test strips for now. Use a piece of opaque material to cover up the print, moving it in increments across the paper surface, to get different exposure values to look at.

(In the link a test strip is fairly well explained, but I would stop the lens down enough so that my exposure was more in the region of 20-40 seconds, which helps you when you start to dodge and burn certain areas of the print. You want to allow yourself enough time to make that happen a little bit slower than the 5-8 seconds you've given so far. It is true that enlarging lenses usually perform best at two stops from wide open, but remember all of the apertures are there to also make your exposure times practical).

Take that exposure calculator out of the equation for now until you know it isn't something else.

I agree with Terry that developing time of at least 2 minutes gives the best tones, but that you may want to adjust that to even longer, like 3 minutes. With experience you will learn to see what happens to the tones when you leave the print in the developer longer.