I read in some grammar book that "If the verb in the main clause is in the present tense we use 'will, can, may' in the adverbial clause of purpose and if it is in the past tense we use 'would, could, might' ", as in:
I write it down so that I can remember it later.
I wrote it down so that I could remember it later.
But what if I did something in the past so that I can do something now? Is it still the same structure? Or is this sentence idiomatic?
I wrote her number down so that I can call her now.