If you want to do the trig, in the modern jets you would need to do the inverse both ways because the system does it for you already. As stated before, BRA gives range and altitude, so put TDC at correct range and set high/low altitude of radar scan to bracket the reported altitude. No need to do the trig stuff. So I'm thinking the real question is, what are the scan zone indications, and how is scan zone adjusted on the fly? Twisty knob? Hotas control? Does the fishbowl show altitude bracket at the TDC, like in the m2k and F15 VSD?
Now with the Su27, it is marginally different. Set the ezpected range to reported range, take reported alt, subtract your alt, and move the scan zone up or down based on the difference divided by 1000 (e.g., you are 5km alt, target at 7km alt, 20km away. Set expected range to 20, and set scan elevation to +2).