This is going to be a long post so be prepared.
Ketotifen is a medically used anti-histamine to treat asthma and allergies. Ketotifen has been shown to inhibit the down regulation of the beta 2 receptors caused by clenbuterol. Consequently, you can continue taking clenbuterol for a longer period of time without having to cycle off to regenerate the receptor sensitivity by adding ketotifen. Additionally, it means that you don't need as much clenbuterol to get the same benefits. Although no studies have been done to find the most effective dose of ketotifen for this purpose, most users find a daily dose of 2-3 milligrams (mgs) to be ideal, and this dosage can be split or taken in one sitting. Meanwhile, higher doses are likely to cause drowsiness and increased appetite that can be sometimes severe.
I will first answer your question and then give you my thoughts on clen.
The best way to use clen is to start off with a low dosage and then increase the dosage every 2 weeks. Your body gets used to clenbuterol very fast so you need to keep increasing the dosage to get the metabolism boosting benefits. I would start off with 20mcg/day for 2 weeks, then increase it to 40mcg/day for 2 weeks, then increase it to 60mcg/day for 2 weeks, then increase it to 80mcg/day for 2 weeks then take a 8 week break and start the cycle all over again.
Clenbuterol is not a hormone so it will not effect the way testosterone or any other steroids you stack with it work.
I personally am not a big fan of clenbuterol.
Clenbuterol is very over rated. Here's why -
Clen simply raises metabolism. It does not differentiate between muscle or fat. Clen might raise your metabolism an additional 100-200 calories per day. You could just easily do an extra 15 minutes of cardio. I have used clen in the past and I always felt it did more harm then good. It caused muscle cramps when I was training and it really negatively impacted my sleep.
There are better alternatives that directly increase fat loss, with no risk of loosing muscle mass and no side effects like insomnia or muscle cramps. Cardarine gw is going to be your best option. It directly increases fat loss, endurance and recovery. It is not a stimulant or a hormone and is pretty much void of side effects.
I haven't used clen myself in years and don't recommend it to my clients at all anymore. If they are trying to burn fat, I recommend cardarine and maybe a good nutrient partitioning supplement like n2slin from n2bm.com.