Controling decimal placed on a formula

I have no experience with coding, but im stuck with a formula for medical equation where you enter the weight and it calculates how much drugs to give…
12kg gives me 1.2000000000002 which i need to only have 1 decimal place
this is the formula… pageVars.child*0.1

anybody got any help… i have tried round but i get that the formula requires an inversible formula

If i understand correct, think this will help you

I have tried that, but it wont work with my formula, keeps coming back as the formula is not inversible

Where are you doing the binding that an inversible formula is required? Could you instead bind it directly to a page variable and do the ROUNDing somewhere else?