Math help (scale down)

I am awful with math and was wondering if anyone can help me figure out how to scale down a 3d print. If you could also explain how so I may be able to figure it out in the future that would be excellent.

I printed something that was supposed to be :

300x300x10 mm

And it auto scaled to :

198x198x6.60 mm

I have some pieces that I have to scale down to match. Can anyone help me please?

Thank you :D