Instalment Tax Threshold?

The 'Income Tax at a Glance' KRA brochure states:

Instalment tax is paid by both individual and corporate taxpayers who have tax payable for any year, except in the case of those individuals whose tax liability for a particular year is fully covered under PAYE, or whose final tax liability is below Kshs 40,000/=

The Individual return has the following section:

PART J - FINAL TAX

49 Total tax payable by wife & self (44B +48A)
50 Total P.A.Y.E deducted (Self & Wife)
51 Credits under Section 12A
52 Other credits or withholding tax deducted at source.
53 Total credits (Line 50+51+52)
54 Tax due/Refund due (Line 49-53)

An individual has some non-employment income, which is not covered under P.A.Y.E. (49)Total tax payable is over Kshs 40,000, but after (50)P.A.Y.E is deducted, (54)Tax due is under Kshs 40,000

For the purposes of Instalment Tax, what is meant by 'final tax liability'? Is it Line 54, i.e. net tax owing after P.A.Y.E and other credits have been deducted? Or is it line 49, i.e. Total tax payable?

I asked my employer's accountant but he didn't sound sure, even suggesting that I wait for a demand notice from KRA!

Thanks for your feedback.