Two ways to improve it
The formula makes one thing clear: availability goes up in two ways. By raising MTBF, so the equipment fails less, with preventive work. Or by lowering MTTR, so it is repaired sooner, with spare parts and organization. The second is usually faster and cheaper.
Watch out for 99%
99% availability sounds very good, and it means about 87 hours of downtime a year. For example, on a production line that brings in $2,000 an hour, that is more than $175,000. The percentage is misleading: you have to translate it into hours and money.
The usual mistake
Committing to an availability figure in an operations and maintenance contract without defining what counts as downtime. You also need to state which stoppages are excluded (scheduled maintenance, force majeure, supply failure) and who provides the data. Without that, the clause is a source of disputes.