Understanding your ideal leftover

Ideal leftover is a percentage of your monthly take-home pay, set on a slider between 5% (tight) and 50% (safe) when you onboard, with 25% seeded in as a sensible starting point. It is not a hard limit the app enforces, you can add a property that blows straight past it, but it is the yardstick every affordability score is measured against.

You can change it at any time in your profile, and every property’s score recalculates against the new target immediately, nothing needs to be re-entered per property.

Say your ideal leftover is 25% of take-home. A property that leaves you with exactly that scores 80 on affordability, and keeping more than your target earns a bonus on top, climbing towards 100 as your leftover approaches double the target. What happens when you fall short is the interesting part, because it is not a straight percentage-for-percentage penalty.

The app works out how close you got to your target as a ratio and then squares it. Land at 80% of your target, meaning 20% leftover against a 25% goal, and you score 51 rather than 64. Land at half your target, so 12.5% leftover, and you score 20 rather than 40. It gets steeper the further you drift, and hitting zero or negative leftover takes the score to 0 outright.

That is a deliberate choice rather than a rounding quirk. The squeeze of being under your comfortable margin does not actually feel proportional in real life, since being a little under target is a mild inconvenience while being a long way under it is genuinely stressful, and the curve is built to reflect that instead of pretending the pressure scales in a straight line.

Numbers make this easier to picture than formulas do. Say your monthly take-home pay is £2,800, and your ideal leftover target is 30%.

A property you are considering comes with a £1,150 monthly mortgage repayment, and an estimated £180 in council tax. Add £430 for the bill defaults the app starts you off with (gas and electricity at £150, water at £45, broadband at £35, and groceries at £200, all placeholders you are meant to replace with your own, deliberately cautious figures), plus your own regular commitments outside housing, which come to £310 all told and cover house insurance at £25, Netflix at £15, petrol at £120 and £150 tucked away into a holiday fund.

• Take-home pay: £2,800
• Mortgage: £1,150
• Council tax: £180
• Bills (gas & electricity, water, broadband, groceries): £430
• Commitments (insurance, Netflix, petrol, holiday fund): £310
• Leftover: £2,800 − £1,150 − £180 − £430 − £310 = £730

£730 is 26.1% of your £2,800 take-home, which falls short of the 30% target. Running that through the curve, 26.1 divided by 30 gives a ratio of about 0.87, and 0.87 squared times 79 means this property scores 60 on affordability despite missing the target by under four percentage points.

That is the curve doing what it is meant to do. A near miss still scores reasonably, but it is visibly marked down rather than waved through, so the trade-off you would be making stays in front of you. Had the same property left you 30% spare it would have scored the 80 baseline, and 60% spare would have earned the full 100.

There is no universally correct number here, since it depends entirely on the sort of margin that lets you sleep at night. Someone on a stable income with nobody depending on them might be perfectly comfortable at 15%. Someone whose income is irregular, or who has a change coming, or who is simply cautious by nature, might want 35% or more.