I can guesstimate smaller and mid size bushes rather easily. It's the big stuff is very difficult to estimate, a 4-5 foot shrub may take 3-5 mins but a 20 footer can take several hours to get it right. Now I do some where I do not know the actual heights but when your standing 10 feet on a ladder reaching so your arms at the 17 foot level with an 11 foot set of trimmers in your hand and struggling to hit the top its a good guess the bush is 28-30 ft tall. Iv done hedge jobs where the hedge is so wide you can not reach the center from either side with 11 foot trimmers. So I know a little about the big stuff I do 50-60 trimming jobs a year and have for the past 23 years. Id recommend charging by the hour creates fewer head aches.
Start with smaller shrubs and hedges and work your way up as you become more skilled and confident.