I advise that first, you gain control of those "hey you, got a sec" tap on the shoulders. They can eat into your time quicker than sands drop in an hour glass! And, you can't measure them!
Ask your Management if you can set a Corporate Policy for "IT Problems". Then, announce this policy company wide. Explain that your time needs to be measured by "documented" sources that top Management can measure.
Your Management should endorse this policy in this communication notice as a "from now on" notice. So that the decision comes from a business perspective rather than some thinking you are too "upity" to be tapped on the shoulders.
Then, when they tap you on the shoulder, let them know that you wish you could, but they must callyour helpline so that you have a logged call for the request because you have to account for your time through that log.
Then, as stated by others, document your every project and include the "thinking" and "research" time. Don't forget purchasing and shipping and receiving (if any). Incorporate these jobs into categories and assign "work time" to them on a daily, weekly, monthly, basis. Re-evaluate as necesary.
Don't be afraid of putting a name on what you do. If talking to the customer is part of the call, then time that, and call it "assessment", if talking to the customer about what you did is part of the call, then call it "post evaluation". BUT, do measure it and account for it. There is "research", and there is "collaboration". These things account for time... So, build them into the equation. You would be surprised at how much time they take up!