Now once the water starts do NOT stop it! Stopping it could cause a line in the new plaster.
Put some socks or such on the ends of your hose to help keep the metal off the plaster. Make sure the hoses are placed right at the main drain to help keep any channeling from the flow.
Take a pic of your water meter as soon as the water starts and then again when it reaches the middle of the skimmer face. Make notes of what other water was used between the start and finish. This will give you are an idea of your size by gallons.