I once tensioned an ACS Z Wheel by feel, the same as I did the aluminum rims before we got a tensionometer. It was perfect... then I went put the tire on. It wouldn't hold the tire bead because the tension was so great for that rim that it shrank the rim.
After you get a tensionometer (and they are only 30usd give or take) you won't build without them. And they will teach you more about good vs bad rims than you ever knew existed.