Can I mix a tube of Retin-A in 300ml of body lotion? I am worried about poisoning due to systemic absorption of Tretinoin.

You can definitely dilute your retin-a, but it will not be of much benefit at that low concentration.  Retin-a is a vitamin A derivative, and the only time we as board certified dermatologists don't recommend it is when someone is pregnant or breastfeeding, in the very unlikely event it could affect the developing baby.  I would consider the reason you want to use retin-a, and perhaps your dermatologist can help you find a formula or some different procedures that you feel comfortable using or having done for your acne or aging skin. Thanks for your question!