How much do I set my travel allowance amount for this tax year?

I should be able to claim a travel allowance as I am consulted out to one or more clients on a permanent basis from my employer. However, I don't have a travel allowance on my payslip and I need on in order to claim. I can request for an allowance to be added, but it needs to be added as a matter of urgency to be part of this tax year. Question is how much do I set the amount to? Would it need to be 12x what the normal amount should be?

Do you keep a logbook for business mileage travelled?
How many kms do you travel a year (approx) for business?
Are you currently reimbursed for travel?

Yes, I have a logbook via my gps tracker.
Travel about 13000km for business (excl personal use)
I am not currently reimbursed for travel, my contracting house say that I can structure my salary how I want; hence me wanting to add the travel allowance

Are you able to suggest an amount? Would I be able to claim for the months that I didn't receive a travel allowance?

There is no set amount for what you should add for a travel allowance amount, but it should be a reasonable amount when related to your salary - how much are you earning each month currently, also what is the percentage of your business to personal travel?

My salary is 70k before tax per month.
I travel about 1100km per month for work. The ratio is about 50% business 50% personal i.e. 1100km is the 50%.

It would be safe to receive a Travel Allowance of about R5000 per month.