The original sector purchase values are of no relevance when topping up, so you have the right idea.
What matters are the current values so that the aim should be to bring any currently undervalued sectors up to the new average value, but not over it, calculated to include the new money. If the cash being added is insufficient to bring any sector up to the new average value, then just add what you can to the most undervalued sector. You also might wish to consider the available yields of those undervalued sectors because if you have to choose between them, you might as well go for the higher yielders first.
Just as the purpose of equal investment at original cost is to avoid under or over weighting any sector, the same applies when topping up or adding new holdings.
Note that only those shares shown as Buys in the latest TDL should be purchased when topping up or adding new holdings.