It is impossible for anyone else to calculate CPH for you since only you have access to your account. You must determine CPH yourself by:
- Killing the monsters for 10 minutes.
- With the amount of charms you have, multiply it by 6 (10 minutes * 6 is an hour).
- That is an approximate CPH rate.
- Repeat the test for statistically stronger results.
A good monster to kill (at your level) is the Wallasalki located in the Waterbirth Island Dungeon.
To get to the dungeon, you can do either the following:
Teleport to Waterbirth Island with the Lunar Magics Spellbook (after Lunar Diplomacy quest)
Talk to Jarvald at the Relekka Docks.
No quests are required to go to Waterbirth Island (and thus the dungeon), but the boat will cost 1,000 gp if The Fremennik Trials has not been completed.
After arriving at the island, go counter clockwise and you will eventualy see a dungeon entrance. There will be lots of aggressive dagannoths at the entrance, so be careful.
There are wallasalkis around the dungeon. A map of where they the monster is located can be found here. According to the unofficial wiki, they have a 12% chance of dropping a blue charm.
Also, every day, you can kill Bork (after completing The Hunt for Surok miniquest). He will always drop 5 blue charms, or 10 if you have completed all of the Varrock Tasks.
I'd like to point out that using blue charms at a low level is not a very good idea. You should keep your blue charms in the future when you have a very high summoning level, and that obtaining blue charms is more common and has a better use. At your level, you should stick to gold and green charms (or maybe even crimsons) to level up.