If you were doing single-ended outputs, it would be easy. You use the equation P = V*I = V*V/R so R = V*V/P

Your 70 volt (actually 70.7 volt) outputs gives you a V*V = 5000 so if you have a 1 watt tap, you have 5000 ohms, if you have a 5 watt tap, you have 1000 ohms between the black (common) lead and the tap.

With 25 volt taps, your V*V is 625.

Push-pull operation gives you the problem of trying to balance the inputs. The black common lead goes to one plate and you have to pick a centre tap and the opposite plate tap. In this case, you can get push-pull operation from the 2.5 watt and 0.625 watt taps but the impedance is too low and although the voltages between the common and 0.625 and the 0.625 and the 2.5 has the same number of turns, the winding resistances will not be balanced. The transformer in your link seems to be drawn wrong. The highest power taps are closest to the common and the lowest power taps are closest to the opposite end from common. See this:

https://www.alliedelec.com/m/d/d8292cf7f11c601de4c651a61586c0eb.pdffor how it should be. Use the tap with 4 times the power of the other plate tap as the centre tap so if you follow the Hammond drawing, you could use a 4 watt tap as the centre tap and the one watt tap as the opposite plate tap and get 5000 ohms plate-to-plate.