Greetings, I'm writing this guide in order to help newcomer combat type Clerics balance healing and combat efficiently.
First, we have to understand the overall public view of Battle Clerics. Battle Clerics are usually shunned, whenever there's a bad cleric in the group they either fall into the "bad cleric" or "battle cleric" category, so the public view of battle cleric and bad cleric isn't very far apart. Of course, it's been argued back and forth that battle clerics are acceptable if they heal properly, but that does not stop others from thinking poorly of the Cleric.
Philosophy
It is important to uphold certain philosophies when trying to multitask between healing and fighting. I don't expect everyone to use or remember them all, but if it helps you that would be great.
1) The party's livelihood determines success: A party that is alive will always have a higher chance of completing a mission than if only certain individuals are alive. As much as a cleric can solo early on, they'll still be much slower in completing objectives than a fully functional party.
2) Full health is great, but enough health is even better: It's not necessary to heal anyone to full, as long as they have enough HP to last long enough for you to heal them again if they need it, it's enough HP. Not only is overhealing a waste of valuable resources, but overall detrimental to what you can do as an individual. The less wasteful you are, the more you can do.
3) The party is wonderful, but you are more important: Chances are, if the only healer dies, the rest of the party will quickly follow after. As great as it is heal that tank in a pinch, if you don't survive there is very little chance of actually recovering.
4) Healing Damage is great, Preventing Damage is even better: A cleric is usually responsible for healing damage taken by party, however the cleric should always figure out methods of preventing as much damage taken as possible. One example is to run up to the opposing enemy group's arcane caster and incapacitate them. It's generally agreed that the enemy arcane caster hurts the most, thus if you can trip/paralyze/stun/sap/etc them, it'll save you enough SP for one healing spell.
5) Mistakes should be forgiven, liabilities should not: Mistakes are common, it happens all the time. In many cases, mistakes result in deaths or major harm. There should never be hesitation to help patch up mistakes. However, there are times when it's not much of a mistake as it is a liability; examples of a liability is a rogue that has died multiple times by using the same tactic of charging into the group of enemy ogres. Remember, you're not there to babysit, you're there to succeed; liabilities hinder your party's chances of succeeding.
6) Those that stand with me are my brothers: The party should ALWAYS be around the Cleric and the Cleric should always follow the larger chunk of the group unless needed otherwise. As important as it is to heal that stray outside the majority of the group, the majority of the group always comes first. Those who cannot understand the importance of teamwork should not receive the benefits of teamwork.
7) Firing the bow is great, but if you can keep the arrow while at it is even better: Instant kills, paralyzing, tripping; you can incapacitate your enemies a great number of ways as a cleric, but if you can do so without consuming SP that's even better. Instead of using that valuable slay living/destruction spell, if you can vorpal/disrupt them instead that'd save SP for other uses. Trip/Improved trip can be used constantly and works frequently against what usually needs to be tripped the most, the enemy arcane caster. If you have a paralyzing weapon at mid-level ranges, you can save a whole lot of SP by preventing damage taken.
I'm sure this doesn't cover all the points, but this is all I can think of at the moment. I might add more as they come to mind, but these are the philosophies that helped me rack up kills, keep party alive, and got things done. Enjoy and I hope I can hear some constructive feedback.