I'm pretty sure this
and also this
answer your question. There isn't any maths that you really can or need to do beyond that.
You're sitting at about 26% bodyfat, which for a female means you're pretty lean. Beyond knowing that about 26% of your weight is made up of fat (and again, some will be visceral and some subcutaneous which will probably make a difference to appearance), there isn't a formula that will let you calculate how much of the remaining 74% of your bodyweight is skeletal muscle.
You could estimate that by finding how much blood you are holding, what your organs will weigh, how much water you're holding and how much your bones weigh. However, it would probably be a wildly inaccurate estimate because all you'd have, if you could find it, would be averages for a human being, maybe female if you were lucky, maybe similar in height and weight to you if you're even luckier. That still wouldn't give you enough accuracy to account for your individual circumstances.
Your best bet, if knowing how much skeletal muscle you have really matters that much to you, is getting a DEXA scan done. I'm pretty sure that would tell you with a high degree of accuracy how much skeletal muscle you're carrying.
I'm just curious, though: I've had a couple of bioelectrical impedance analyses done recently, and I got various data including skeletal muscle mass (also bone density, visceral fat percentage, total body water, etc). At 88 kg and 23% or so I had just over 43 kg of skeletal muscle mass, so I'm wondering why you weren't given similar data.