The $25,000 Concessional Cap includes the sum of your employer Super Guaranteed Amounts (currently 9.5%), any Salary Sacrificing Amounts and any Personal Contributions Amounts you make and claim a Tax Deduction for in your Tax Return.
If your Concessional Contribution exceeds the $25K Cap, you may have to pay extra tax.
In order to make sure you don't go over the Cap, you will need to keep track of your employer's SG amounts plus any amounts you salary sacrifice. As your salary might be a bit of an unknown until you get closer to the end of the financial year, this might be a bit hard to keep track of.
A better way might be to make personal contributions into your Super Account on a regular basis (say $1500 to $2000 per month) instead of salary sacrificing.
Then once the end of the financial year has passed you can work out how much of these personal contributions you wish to claim a tax deduction for.
Say for example, at the end of financial year your employer SG Contributions are $10K. You have regularly contributed $1500 per month in personal contributions ($18,000 for the year). Then you would contact your Super Fund and tell them you would like to claim $15K out of your $18K personal contributions as a tax deduction. They will then send you a confirmation letter so that you can then claim the $15k as a tax deduction in your Tax Return. You will always need the confirmation letter from your Super Fund before you can claim the deduction.
In the example above, the additional $3000 of your personal contributions will remain as non-concessional (or after tax) contributions (for which the Cap is $100k per year). This way you are certain to not exceed any of the Caps, and not be penalised with any extra tax.