Clenbuterol use results in rapid down-regulation of beta 2 receptors. This is due to the powerful stimulatory effect of the drug. It is therefore pointless to use clen for long periods without a break. I like a two week on, two week off cycle. Maximum plasma levels are reached around 2-3 hours after oral administration, and terminal half life at 34 hours. Clen has a long half life, resulting in continued stimulation even throughout the 'off' days. Clen also has the nice effect of being anti-catabolic.
So to put it all together .... you could start with 20mcg on day one and up the dose 20mcg per day until you reach a peak of 140mcg (on day 7) ... then hold 140mcg per day for the next 7 days. take 2 weeks off and repeat process. I have done this for up to 12 weeks. 2 on, 2 off, 2 on, 2 off, 2 on and 2 off. There is no need to to exceed 140mcg due to receptor over-saturation.
Of course this is all dependent upon user toleration as well. I have known guys that could not tolerate Clen. That is why you start at 20mcg and ramp your way up from there. Clen has its own set of side effects that you should be aware of. They include headaches, muscular tremors (especially hand shakes). muscular cramps, nervousness, insomnia, sweating, increased appetit, nausea, palpitations, and hypertension (high blood pressure). If you have cardiac and/or hypertension issues ... don't even touch this stuff. Not worth it.
Cramps, insomnia and sweating were the big ones for me. Hydration and bannanas seemed to help with the cramps which could be killer for me at times.
Adding T3 to your clen cycle is also advisable but you didn't mention that so I will save that.
To be honest, have you even tried altering your diet and throwing in some fasted interval cardio in the morning to drop the bf% you desire???
I think you will find that to be a much safer and can be just as effective if not more so if done correctly.
All of this is just opinion .... not saying you should do any of the above. Just my experience.