i am not really sure why they might have left the value out,sometimes when we write out papers we miss out on some basic information which makes sense to the author of the paper but not to everyone else.
There is a way of finding the exact distance though. by using a paper and protractor.follow these steps and you will be able to find the exact distance.
draw an another square on the same one but with a projection so each side is 30 cm.
now mark the centre of the semi circle as the centre of the circle and square, project the semi circle to make it a circle.
now measure the distance from the smaller side(centre of the side) to the edge of the circle in a stright line.which should be 1.5cm's.
Meausre the difference between the distance of the circle from the 25 cm side and 30 cm side and you should be able to figure out the distance.
thats the best way i can come up with.
Regards
Rotmanlens