When determining percentage of annual savings vs annual income, should one factor in annual bonus or should bonus be outside of the equation?

I believe what you are looking for is your Savings Ratio. This measures your ability to save money after factoring your income & expenses. What you can do is to take your total cash surplus and divide it by your total cash inflow. For example, if you have a take-home income $2500 and a remainder of $1200 at the end of the month, you can take $1200 / $2500 = 0.48

From this, you can tell that you are saving roughly 48% of your take-home salary. A good rule of thumb is that you save at least 10 ~ 20% of your monthly salary.

As mentioned in the answers below, there is no hard and fast rule and it's worthwhile to take the time to figure out your expenses to figure out the percentage of your income that can feasibly be saved.

No hard and fast rules...

But if you are thinking of fixing X% of annual income to save.... may I suggest to consider listing out all necessary expenses food/transport/rent/bills/tax/insurance premiums etc to look at what % of annual income is spent on necessities and wants first.
With that on the table, one would have a better sense of what % of income could be saved rather than deciding on an arbitrary saving rate. This will avoid the scenario of setting an unrealistic arbitrary saving rate and help one re-examine whether he is spending too much on an item or in fact, can actually afford to spend a little more in certain areas.
In the above scenario suggests one to be prudent and exclude year-end bonus as part of your annual income. It feels like there's a looming recession, so it might be safer to be on the conservative side and treat the year-end bonus as a bonus when the paycheque comes.

On the other hand, if the intention is to look back and track your past spendings and savings. As much as possible one should include all expense and income estimates (be it in the form of bonus or additional allowances).