Concrete Interference?

I use the MLM2PRO outside on a concrete deck, hitting into a net; there aren't any concrete walls around or anything. Does anyone know how much that messes with the readings? Any midigations? I've been laying a towel down under it (not sure if it works), also the concrete panels are slightly sloped (nothing crazy, just for water drainage), I figure that might also create some problems. If anyone has any thoughts im all ears. Thank!