You’re correct, if you lower the front without lowering the rear you lose some positive caster. I haven’t checked your math or anything but it’s a fairly minor effect as long as you don’t run a massive rake on the car.
How wide are your front tires? With just the offset bushings that’s the only thing I could see being an issue. The wider the front tires are the more positive caster is helpful to counter the effects you get from really wide front tires. And with just offset bushings your ability to dial in more caster will be limited. My set up is a bit extreme, but with 275/35/18’s on the front of my car I like to have +6° to +6.5° of caster. Which is more than you can get with the offset UCA bushings regardless of any rake.
