Like any tool, it's how it's used. The more 'correct' information given, the more acturate it will be. But some of that 'correct' information you won't have until you complete the actual build. We all would like an even 11.0:1 compression, but it could actually be higher or lower depending head gasket compressed thickness, the 'real' volumn of cc'd heads, knowning the real flow numbers, etc.

You can put in the numbers you want, but what ends up in the vehicle can be completely different. My past experience with an older version with 4-cylinde engines came to within 3%-5%. This could be a variation of 5-10 HP.