I am also one of those that has the saddle very slightly angled up.
I don't think you should adjust fit by moving the saddle around. Set the saddle up in relation to the bottom bracket and keep it there. Then adjust your stem-height and reach. You may be sliding forward due to your seat angle, but also if your bars are too low for you. Depending on your core strength, you may not be able to hold your position well if the bars are too low. You can decrease the saddle to bar distance by raising the stem, not by lowering the saddle.