that is why probably ikkyo is the most important and most difficult technique in aikido because it relies purely on timing. You can only make it work if the timing is perfect.
"Ikkyo" can be done with any timing, sen no sen, go no sen, or sen sen no sen. Timing is merely a way of describing when, in the duration of uke's attack, tori is applying "ikkyo". The only correctness in using timing is when you are looking for a specific variation of "ikkyo" that looks a specific way in relation to a specific attack.