Learn to heal first then learn where you can fit dps in. You will find lots of down time in healing in trials where you can dps. Being able to judge how far your mana will stretch just comes from experience. In dungeon runs, esp 50+, you will find you can contribute a lot of aoe dps as a healer, making the run far faster. As a WHM your main aoe will also stun mobs, making it so very little healing is needed in a dungeon if you use your aoe dps rotation.

It is commendable that you are already trying to figure out how and where you can dps as a healer. The truely skilled healers in this game can oush out a filthy amount of dps whilst keeping everyone alive. .