how do i flip an arc length dimension? 

in the attached picture that 261mm dimsion is the outer arc length dimension. i would like to flip it to read the other side of the arc which is about 144mm.

the funny thing is when i start off dimensioning it it shows it as 144mm but when i click ok and place the dimension it changes it to 261mm.

i am trying to use it to form an equation but if its showing the wrong value it makes things a bit harder.

also does anyone know how when dimensioning a sketch to make the dimension driven straight away rather than doing ti twice waiting for it to come up with that error message warning you of an overdefined sketch then deleting the driving dimension?