you can use a value of 4 hours, it’s quite standard. Since you have a low amount of subscribers, I doubt you’re looking to offload your MSC/VLR.
To increase your cssr… mmm. it depends if you take the paging success rate into account.. probably not.. but for sake of argumentation : if yes, then you could try to reduce the T3212 a bit, see if it changes anything, but you must also reduce the implicit detach timer in NSS :
Some (most ?) mobiles do not send an IMSI DETACH when their battery runs off. The Periodic LU will force the MS to send a LU every T3212. If the MS doesn’t do it, then the NSS will perfom an implicit DETACH, based on a certain NSS timer (t-something).