Bish -- I mix my own RLM 74/75 and 76 in Tamiya Acrylics -- here's my blog post that discusses the ratios. I'm still using the same ones today, I did the Fw 190 A-8 last year using them (though I have substituted MM Acryl 76 as it's spot on -- but their 74 and 75 are miles out.)
"Their [Tamiya's] formula for RLM 76 (XF-2 Flat White plus XF-23 Pale Blue plus XF-66 Light Grey at a ratio of 7:1:2, provides a virtually perfect match for the original, lacking only lustre, and an addition of 25 to 30% X-22 Clear Gloss approximates the sheen of the original RLM colours. Unfortunately, their suggested formulas for the 74 and 75 are miles off, and I experimented to find a decent fit. For 74 they recommend XF-24 Dark Grey and XF-27 Schwartzgrun at 3:2, but this was too dark and too green, especially compared to the Model Master enamel equivalent. I reduced the ratio to 3:1 and added 1 part Flat White as well, then brought up the lustre with 30% Clear Gloss. For the 75, Tamiya suggests XF-51 Khaki should be a component, but as 75 has no green hue at all this mystifies me. I mixed it from scratch, starting with XF-24 Dark Grey and adding Flat White at a ratio of 2:1, then adding 30% Clear Gloss. On reflection this was too much gloss, as darker shades reflect more strongly than lighter ones, and 20% would have been ample. The final shades were acceptably similar to the Model Master enamel equivalents, though I have to say the acrylics are both more delicate and lack a certain 'gutsiness' that the solvent-based paints have."
Cheers, M/TB379