For 18 weeks cycle, I wouldn't use HCG in a middle of the cycle, it's not really that long. In a middle of the cycle HCG is nessesery, if one is on for really long time.
First week after the cycle is a good idea to use HCG,but only with Clomid and continue with Clomid. HCG will jump start your testicles, and even it's not going to be really your natural testosteron production, but then Clomid will pick up and I really doubt that you will need more then 3 weeks of Clomid.
The best thing to do, would be to run blood test after 3 weeks of using Clomid, that would be the most accurate judgement of should you stop or not.
If not blood test, try to evaluate your recovery progress by checking how much fuller your testicles are, and how much more sperm you produce.
Also, 300mg on day one and continue with 50mg for 3 weeks sound like the best way to do it right.