Making triplet equal eighth note in metric modulation?

And then do the math :-)

First thing to know is that MuseScore always consider quarter notes to be the definition of beat as in BPM, so in your 12/8 bars you have six beats per measure. (but we all know that you actually have 4 beats, but for BPM calculation you need to think about it as six beats.

if you have x BPM in the 12/8 part, that means that you have

x * 2 8th per minute, or 1/(x*2) minutes per eight-note

if y is the tempo in the 4/4 bars, then you have

y * 3 triplet-eigthts per minute. or 1/(y*3) minutes per tripplet eight-note.

Now you want the triplet eighth-notes in 4/4 to have the same length as the eighth-notes in 12/4. That means that you say that
1/(x2) = 1/(y3)
Which means that
x = y * 3/2
y = x * 2/3

So if you what the BPM of the 4/4 part (y) to be 100, the BPM on the 12/8 part should be 100 * 3/2 = 150 BPM