at 20% BF there is NO REASON you should lose any lean tissue while on a diet unless you do something really stupid.
1) find your maintenance caloric intake
2) reduce that number by 20% or so.
3) multiply your LMB (in lbs) by 1.5, use that number to determine the grams of protein per day.
4) multiply the number of grams of protein by 4, subtract this number from your total calories. This is the amount of calories you can get from fat and carbs, split it up as you want (perhaps 50% carbs, 50% fat)
so if you're 98 kg at 20% fat the calculations would look like this:
1) maintenance intake (BWx15)= 3,235 calories
2) 3,235 x .8 = 2,600 (rounded up)
3) 172 lbs LBM x 1.5 = 260g protein
4) 2,600-(260x4)= 1,560 calories from carbs & fat. Split evenly, that equates to ~200g carbs, and ~85g fat
track your calories closely by using any of the dozens of free calorie tracking websites, and adjust calories up or down based on real world results (up if you're losing strength too fast, down if you're not losing weight), aim for a loss of 1-2 lbs per week.