Performance Monitoring Traffic throughput

Using a LTM 1500 on 9.4.3. Looking to understand when we are in need of upgrading to a larger LTM device due to traffic throughput. From a spreadsheet provided with all models of LTM's the following are performance thresholds against the 1500 LTM :- L7 Connections per Sec (1-1) 5k cps L7 Requests per Sec (1-inf) 22 rps L7 Requests per Sec (inf-inf) 55 cps L7 Throughput 500 Mbps But when I ask what these actually mean and how I report on these so that I know when I need to upgrade a device based on these. I get no sensible replies. I am using SNMP/OID to report on HTTP current connections but am looking for some one to tell me what to monitor to allow me to know if I need to upgrade. I am hoping someone will be able to help.