Ok the thing with what you are talking about is active levels of the RAW hormone. 100mgs of boldenone undecanoate aka EQ actually doesn’t do anything on your body until the liver removes the ester. So the level that would determine if natural production is even possible is the amount being released from the ester per day.
For my example I will just keep it simple and say we are only dealing with testosterone enanthate. Now your body naturally senses when it’s testosterone levels are too high or too low. You body also can not determine if the testosterone is synthetic or natural. So if in a normal natural state your body wants about 4mgs of test per day but for some reason only has 3mgs then it will produce/release that extra 1 mg. Now fast forward to and through a 12 week testosterone enanthate ONLY cycle. Say we used 500 mgs per week and stopped injecting on the last day of week 12 and for this situation we are not calculating the build up or anything, just 500mgs on the last day and that’s it. So a week goes by (we are saying enanthate has an EXACT half life of 7 days for this example) and at the end of the week 13 we have 250mgs of test enanthate still I our system. For week 14 we will chew though another 125mgs of test enanthate. Even with ester weight and all that it is possible that by the end of week 14 our per day amount being released from the ester is low enough for natural production to try and make up the difference but really it’s probably going to need to be lower. So begining of week 15 we still have 125mg of test enanthate in our system and doing the math we will chew through 62.5 mgs that week. Keeping in mind that each day will have lower and lower amounts being released from the ester somewhere in week 15 our per day amount being released from the ester should be lower than the bodies desired amount of 4mgs per day. So in theory somewhere in week 15 our natural production could kick back on because our body senses that it needs more than what is there.
Now I doubt that the exact day that our synthetic test levels drop below 4mgs our bodies start producing the needed amount to make up the difference. However somewhere shortly after our per day amount being released from the ester drops below that 4mgs it would be a good idea for us to be using SERM therapy to help kick start the whole process of the HPTA loop.
One last thing to keep in mind when reading this example or thinking about your PCT start, remember to calculate for the ester weight. With 100 mgs of testosterone enanthate there is only 70mgs of raw testosterone. So 6mgs of test enanthate is only about 4.2mgs of raw test.
Now when we are dealing with other hormones, idk how that all plays out. If the hormone is DHT based I would think the body produces less reductase enzyme unless the level is so high then it stops producing test all together. For something like EQ which is test based, I would think the body would most likely just think that it is test but IDK, I am not a doctor. For something like tren or deca, since nandrolone does naturally occur in the body (at very tiny levels) then I think even small amounts could possibly be totally suppressive in some guys.
When timing PCT after EQ the things to remember are, ester weight; 100mgs of eq only has something like 64mgs of raw boldenone. The second thing to remember is it’s ridiculously long half life. So if there is a 500 mgs at the beginning then a half life later aka 2 weeks there is 250mgs left. So over the next half life 125mgs will get chewed up, but what is that averaging per day during that 14-15 day half life? Also keep in mind that the “average” being released for that half life is only going to be sort of accurate in the middle of that half life. Each day is less and less being released from the ester. Then calculate the raw hormone weight.