Hey Jim - first of all, my pic was a guitar in course of construction, just to make the point - it is nowhere near the finished standard which will be a hell of a lot better, I spend months on each guitar getting details right.
The offfset issue arises if you use a bushing, it's logical and easy to calculate it. Once you know what size rectangular hole you need, you make the template. I find the easiest way to make an accurate rectangular-hole template is to run a strip through the table saw, set to the short side of the rectangle. Chop that strip with an accurate chop-saw, space the two cut ends apart the length of the long side, and glue additional strips on either side. I use scraps of laminate flooring for those, and I get to within 0.5mm of the size I want usually, at the initial stage. I use masking tape to fine-tune the template down to the final dimension.
The radius at the corners is governed by your cutter - you do a trade-off between fast removal of material and small radius. A planing or bottom-cleaning bit will cut quickly and leave a clean flat surface on the floor of your routed area.