First off, why do you think you need to use Tylosin (Tylan)? Have you identified a specific pathogen that Tylosin is known to be effective against in reptiles? I haven't seen any tests regarding the use of Tylan for treatment of RI in reptiles. It is normally used in cattle as a treatment for irritation of the GI tract and for colitis in small furries, and I think for it's apetite stimulant properties at times more than for it's antibacterial properties. Normally a preparation such as Baytril is used for RI in reptiles. I have no idea what the dosing would be for Tylan in snakes.
If you have identified the pathogen and know that you need to use Tylan, then you are going to need the exact weight of the snake. The dose will be measured in mg/kg. Then you need to know the strength of the preparation you are using. From that you can calculate the amount to give, but you also need to know how many doses are required to be effective, over what period of time. You also need to know something of the pharmacology of the drug you are giving..what is it's half-life in reptiles? What is the accumulation factor? What levels of accumulated drug are likely to produce toxicity? What are the possible side effects and how do you recognize and treat them?