I'd sure run a test exposure sequence first from zone 0 or even -0 through zone 11 or so, write the numbers on the frames with a sharpie, and make a contact print at the minimum time needed to get the edge of the film to equal the pure black of the paper - using my standard paper. That will give you an idea. Otherwise, I wouldn't know what to suggest. That's what I do, and it doesn't matter whether I'm close or not. I just change as needed, test again, and start using it in practice as I tune it.
If you are using cut film holders, you can use the darkslide to progressively double your exposure by moving it in increments. This was proposed in the first edition of Minor White's Zone System Manual. It does introduce inaccuracy through the intermitency effect, but for starters it will be adequate.
I've used dektol that way in the past, but with the addition of acetone. That increases the pH sharply. I didn't use it a lot, and it was so long ago I don't remember much except that it was a very decent developer. There was a fellow in the Bay Area in the 1960's named Don Ross who I understand used it, and he had some standing as a technical expert at the time; I still remember his images, and they were fine, too. The acetone breaks the sodium sulphite up and you get NaOH.