Wort dilution

[Redbloke]

Other than using a hydrometer, is there an easy way of working out the effects of wort dilution? For example: if my three gallons of wort is measured at 1.054 and I add one gallon of water, what will the new reading be?
My initial guess was minus 18, but this looks too simplistic

Cheers

[Redbloke]

... that's excellent Daab, thank's very much

[anomalous_result]

You have to take a weighted average. There are 3 lots at 1.054 and one lot at 1.000 = 4 total lots. So i'd do ((1.054*3)+(1.000*1))/4 = 1.0405