Calculating absolute values from cumulative data set

Thanks  ryan_mayu . That has worked but I realised  I left a fundamental part out. I will have different sites so want to apply this calculation per site i.e so I get the january absolute value for site 1 and site 2 (i will have lots of sites in reality) is there a quick edit to account for this?

Using current formula it goes a bit wrong

Thanks for your help

James

Cunners123 

pls try this

Column = 'Table'[cumulative]-maxx(FILTER('Table','Table'[Month]=EDATE(EARLIER('Table'[Month]),-1)&&'Table'[site]=EARLIER('Table'[site])),'Table'[cumulative])