Standard Radius Curves and gradients

I am back to track planning as I now plan to hae a bigger room - whaay!

Reading various articles, people often talk about standard radius curves in terms of a reference for tightness, e.g. in terms of what couplings can cope with - I have not been clear on what these equate to - so for my own reference here they are -

1st radius - 37.1cm
2nd radius - 43.8cm
3rd radius - 50.5cm

I have also been investigating gradients - the general consensus is that 1 in 25 (4%) is steep 1 in 50 (2%) ideal, I am planning on around 3% so I think I may get away with that.