Aim to maintain your pool water level a minimum of halfway up the skimmer box opening. Any higher than this opening will prevent the skimmer box from drawing in floating debris. Any lower and your pump could be at risk of running dry and will hinder the performance of your filtration system when your pool needs it most.
An average suburban outdoor pool relatively sheltered from hot winds would be expected to lose no more than 10mm of water per day during summer which could be up to 40 litres daily in an average 50,000L pool. Rural and semi-rural can expect a much greater loss, and figures of 25mm or more daily are not unheard of.
If you notice a greater loss than that, you should consider the possibility of, and look for, a leak in your pool or in your pool's plumbing system.
Be careful when topping up your pool to observe any water restriction guidelines applicable, and if your pool has no cover, arrange to have a solar or thermal blanket fitted to prevent up to 97% of water evaporation and also save money on heating costs.
After considerable rainfall you may need to lower your pool water level. If you have a sand filter you can select the "WASTE" function, or in the case of a cartridge filter any pool builder should not neglect to have installed a 3-way valve between the pump and filter with provision for a waste outlet to perform the same task.This excercise of removing excess water may take up to 10 minutes depending on pump output and plumbing size.