When buying to let a common practice is to calculate the percentage yield a property will generate you. This refers to how much of a percentage of the property you will earn back on your investment every year renting it and can be calculated by taking the yearly net rental income (your rent minus any management fees, rates or bill) and dividing it by the total money spent in getting the property and bringing it to market (purchase price, stamp duty, solicitors fees, estate agents fees and any costs for works potentially done to a property) and then multiplying the result by 100. The average accepted rental yield that most people look to get is generally at least 6%.
If getting a buy to let mortgage it would also generally be recommended to take on a property only ever if the amount of money being earned will pay for your mortgage payments.
Of course, it can also be important to consider the average appreciation of property value (on average from 2011 to 2021 property has gone up I value at a rate of 2.6% a year in Northern Ireland) as well as if you feel like you are buying into an area which may be generating lower rent now but could become more desirable and generate higher rent in future.
Given the current level of financial volatility in the markets and the impact on interest rates it is always prudent to allow a buffer each month for increasing interest payments.