The resistance of the secondary circuit is very low and is not the most important factor limiting the secondary current.
In this kind of transformer with primary and secondary wound byside, the short circuit impedance is high (more or less 20%) and the inductance X2 will be the most important impedance limiting the secondary current.
In my opinion, the only way is to measure all the parameters of the transformer and eventually modify the number of turns to obtain the secondary current you want.
First, you should know what is the nominal primary current of your transformer. There are 2 models of MOT, 800W and 1000W. How much power is your MOT ?
Nominal primary current is :
for 800W and 230V: 3.48A
for 1000W and 230V: 4.38A
For short time, you can overload 2 or 3 times the primary of the transformer.
Then, you make the secundary with isolated big wire using all the space of the window.
Short-circuit the secondary and feed the transformer with a variac starting from 0V.
You increase the primary voltage, mesuring primary voltage, primary current and secundary current. (use a current clamp)
Don't overload the transformer, stop when you reach the primary nominal current.
Report the values here.