A no-show is not a gap in the diary. It is a slot you turned other people away from.
Most businesses that start taking deposits do not do it to collect the money. They do it because a customer who has paid something has made a decision, and a customer who has only picked a time has not.
Start smaller than you think
The instinct is to ask for half. The number that works is usually much lower — enough that forgetting is inconvenient, low enough that nobody hesitates at the last screen. For a £40 service, £10 does most of the work.
Ask for it on the services that hurt
A fifteen-minute appointment that goes unfilled costs fifteen minutes. A four-hour one costs the afternoon. Deposits are set per service in VoltsBook for exactly this reason — put them where a gap is expensive, and leave the quick jobs frictionless.
Say what happens to it
The single biggest cause of abandoned bookings at the payment step is not the amount. It is not knowing whether the deposit comes off the final price. Say so, in the sentence next to the button.
