Uptime is not about uptime. It is about downtime. That is what you need to be measured: the exception, not the norm.
Do not go with just simple percentages of uptime. Repeated short downtimes may be more of a problem for your organisation than one occasional one. Alternatively, you may be able to tolerate brief outages but one long ones may be disastrous. So the number of instances and duration should be specified too, according to the actual impact on the business. If you don't care that the system will be down every day from midnight to 6 a.m. but need it up the rest of the time then say so, don't agree to 75% uptime.
Also specify what is acceptable to you as the number of instances and the duration of each instance for each period under measure. You can specify limits per day, week, month and year if you wish: is is your contract and you are the customer. Don't forget you can use rolling periods too (so a downtime at year end and, straight after, a downtime at years start are considered together as a problem in a short period despite being across a reporting period boundary.
1 hour per year
E.g. "We will allow you an hour a year downtime to test the UPS or similar business continuity system, find fault with it and get the system available again. No other downtime is acceptable".
1 hour downtime per year is 99.988 584% uptime (it needs that many digits to be accurate to the second).
- 99.99% uptime is 52½ minutes per year downtime.
- 99.989% uptime in a year is 57¾ minutes downtime.
- 99.988% uptime in a year is 1 hour 3 minutes downtime. This is what you are after when permitting an hour's downtime per year.
- 99.98% uptime is 1 hour 45 minutes per year downtime.
1 minute per week
E.g. "We will allow up to a minute downtime every week to determine the weekly switch to the mirror site has failed and to switch back, or for the weekly switch to be completed."
1 minute downtime per week is 99.990 079 365% uptime which is near enough to 99% to call it 99.99% uptime.
If you will tolerate about one hour per year once per year and also one minute once per week, then say that. Don't just put 99.99% and 99.99% together to get 99.98% That could be used by the supplier as one two hour downtime. Be explicit about what the requirement is.
The period under measure
A common trick is for suppliers to carry over uptime (or rather, an absence of downtime) from one period to the next so that when the contract says "99% uptime" meaning with a monthly review period, they will use the continuous uptime since the start of the contract, calendar year, financial year or quarter to show they are actually within the agreed limit when averaged across that period. If you want 99% uptime each month, then measure each month individually.