The geometry of a chamfered corner looks like this:
Angle ABC is equal to θ (the outside angle of the chamfered corner). This is bisected by BD, so angle DBC is equal to θ/2. That means the offset distance d from the corner to points C and A is equal to r × tan(θ/2).
For a right angle, θ=90° and tan(θ/2)=1, so d is equal to r.
As Robert Longson said, you would be lucky to find an SVG application that exports chamfered corners using circular arc segments. However, with a bit of effort, it should be possible to detect and convert them programmatically.