Drop the 1.0 off your sg and make the last digits a number, in your case 50 for initial gravity and 40 for final gravity.
The maths is easy. The amount of fermentables in your brew doesn't change.
initial volume x initial gravity = final volume x final gravity
rearranging gives :
initial volume x initial gravity/final gravity = final volume
plugging in the gravities gives :
initial volume x 50/40 = final volume
initial volume x 1.25 = final volume