Most CG formulae are designed to produce a model with a fixed amount of Stability from whatever size of tail they use. I can't speak for the one on the site you quoted, but the formula I use tries to give a Stability Factor (Kn or dCm/d-alpha) of 0.15, that's 15% of mean chord.

My simplified formula, expressed like the one on that website is
CG position = 10 + 40*Vbar as a % of mean chord
where Vbar is the Tail Volume Ratio (the standard symbol written in text books is a Capital V with a bar over it, but that's hard to type so I write V-bar, as it is said).

I said that's the simplified form, which works for your average sort of power model with wing Aspect Ratio (ARw = 5 to 8).

To allow for a wide range of Aspect ratio my full formula is
CG = 0.1 + 0.25*(SQRT(SQRT.ARw))Vbar as a fraction of mean chord (x100 for %)
You take the wing AR, take its square root, hit square root again (to get the fourth root), multiply by 0.25 and by Vbar and add 0.1

All these formulae work the same way. They calculate how far the Neutral Point of the whole aircraft is behind the wing's Aerodynamic Centre (that's the second part of the formula, the bit with Vbar).
Then they add the distance of the AC behind the mean chord LE (0.25 or quarter chord or 25%) and subtract the Stability Margin you want (0.15 or 15% chord in my case).

That website seems to cater for Vintage or Free Flight models?
For regular powered Sport or pattern models Vbar is usually in the range 0.5 to 0.8
For scale models it is often from 0.35 to 0.6
Gliders tails are very often very small, but the long tail arm gives Vbar of 0.3 to 0.5

AS Tall paul suggested, measure up some successful models and see what Tail Volume works on the models you like.
Alasdair