Jane,
If you can negotiate teaching jargon, then a good place to start might be the Maths APP grids. Look at the Level 2/3 and level 3/4 ones.
nationalstrategies.standards.dcsf.gov.uk/node/20683
Basically, if you look at the satatements in Level 3, if your child can do some things in all the columns INDEPENDENTLY AND REMOTE IN TIME FROM DIRECT TEACHING OF THAT SKILL then they are working at a low level 3 (3c) (possible also if they are very good in some columns and have very little in others - for example some of my class are excellent at data handling and are working in level 4 for that, but in knowledge of number facts they are relatively weak and have achieved only all the Level 2 statements for that).
If they can do everything in pretty much every column (bit in capitals applies here too), with a few gaps in indiviual items here and there - they are a secure 3 (3b)
If they can do everything in every column (insert bit in capitals again) and a few things in Level 4 - they are a high 3 (3a)
[It's more subtle than that to arrive at an overall level, before a teacher shouts at me for getting it wrong - but in layman's terms that's pretty much how it works]
I have always found the fact that 3c is lower and 3a higher confusing... but that's just me.
If a child can do e.g. mental addition of 2 digit numbers during the week that it is specifically taught, but cannot do that a few weeks later, then they are counted as NOT having achieved that - it's really important that they can do it independently and not directly after being taught.