Airsupply lets you set your own day rate - and to differentiate between short-term positions (generally without much planning, staff meetings, parents evenings and similar) and long term positions (which may include additional work expected above and beyond lesson time).
The principle is that you should be able to take home a little more working through Airsupply than a traditional agency. You may want to consider what you are being paid by a current agency and set your day rate a fraction higher - or choose to add the teachers' pension scheme, should you wish.
You can set your day rate at any rate you would like but it should reflect the fact you will not get paid for days you do not work. If uncertain, a different way of considering your day rate is setting it according to the annual salary you would be able to get in a school with your level of experience as a teacher. For example if you have been qualified for three years you could base your rate on MS3 salary and any weightings usually payable e.g. Outer London waiting.
MS3 without any weightings is £26192 p.a. so per day it is 26192/195 or £134 a day.
If you were on the upper pay scale and you are working in a "regular" supply teaching position, it may be hard to achieve your payscale, if the role itself is not an "upper pay scale" role. However, you are free to set your pay rate as you wish.
It is also worth considering that the school will have a higher cost for you if you choose to benefit from the teachers' pension scheme, as your total cost may affect the school's choice of teacher.
If you are being paid through the school payroll the school will be charged your day rate, Employers NI, and Employers pension contribution if you have opted into the Teachers Pension Scheme as well as the Airsupply fee.
If you are raising invoices as self employed or through your umbrella company (should you wish to use one), the school will be charged your day rate plus Airsupply fee for your services.