Substraction
When working with ratio's, do not add / subract.
It's more like:
octave:1> 240/300*37.5
ans = 30
Your calculation is off:
octave:2> (300 - 70 ) / 1.25
ans = 184
octave:3> ans+60
ans = 244 (Not 240V as you thought).
A long time ago (25+ years) I saved a 0.7A variac from the dump.
Combined it (wen I was still young) with a 2x40V transformer and a bunch of diode's / elco's / fuses / switches to make an un-regulated variable (AC & DC) power supply.
Works beautifully (Within it's limit's).
I have also used this for measuring the characteristics of unknown transformers.
With this power supply I can (safely, the 4x40V transformer's secundary is isolated) put a variable AC voltage on an unknown transformer winding, and make some notes while slowly increase the voltage.
Once you get near (over) the working voltage for the unknown transformer the current increases non-lineair.
The biggest disadvantage of running a transformer on a voltage lower than it's rated voltage is that a part of the iron is "not used", but the weight is still there.