If it encounters the string value (2.5 MG/3ML) 0.083%, which part do you actually want? The 2.5? If so, you could try and do a replace over the different characters you want to exclude.

Below example replaces the characters "(", ")" and "/" with nothing, so basically it is removing those characters.
tonumber(replace(replace(replace(split({table.field}," ")[1],"(",""),"/",""),")",""))

Testing this on your specific example, I get 2.50 back