The standard weekly rate of SSP is £75.40 at the moment.
but the first three normal working days you are sick don't count which is why you may think it is lower.
It is paid proportionally for the days that you work, so if you work 1 day a week and are ill for 4 weeks, you get nothing for the first three weeks and £75.40 for the last day.
if you work 5 days a week and are off from monday am until the next monday, you would get paid 2/5 of a weeks SSP £30.16.. if you normally work 6 days a week, you would get paid 3/6 or 50% of £75.40.
Sorry I used to have to check this all in employers records, and fight with employers to pay their employees what they were entitled to.... there are also linked periods but that gets more complex.