How far is attachment allowed?

This is not a direct answer per se, but I hope that you can find an answer within it. Acknowledging the absence of a dear one and remembering their good qualities are different things from dukka. It would be a good meditation to see at which point this acknowledgment & remembrance turns to dukka, and investigate the causes - you may find that this dukka is due to remorse, broken expectations, etc. Yet for one who has nothing to feel remorseful, has no expectations, the absence is felt, but there arises no dukka.

Ukkacela Sutta

Once the Blessed One was dwelling in the Vajji country, at Ukkacela on the bank of the river Ganges, not long after Sariputta and Maha Moggallana had passed away. And at that time the Blessed One was seated in the open, surrounded by company of monks.

The Blessed One surveyed the silent gathering of monks, and then spoke to them, saying:

"This assembly, O bhikkhus, appears empty to me, now that Sariputta and Maha Moggallana have passed away. Not empty, for me, is an assembly, nor need I have concern for a place where Sariputta and Maha Moggallana dwell.

"Those who in the past have been Holy Ones. Fully enlightened Ones, those Blessed Ones, too, had such excellent pairs of disciples as I had in Sariputta and Maha Moggallana. Those who in the future will be Holy Ones, fully Enlightened Ones, those Blessed Ones too will have such excellent pairs of disciples as I had in Sariputta and Maha Moggallana.

"Marvelous it is, most wonderful it is, bhikkhus, concerning those disciples, that they will act in accordance with the Master's Dispensation, will act in according to his advice; that they will be dear to the four Assemblies, will be loved, respected and honored by them. Marvelous it is, most wonderful it is, bhikkhus, concerning the Perfect Ones, that when such a pair of disciples has passed away there is no grief, no lamentation on the part of the Perfect One.

For of that which is born, come to being, put together, and so is subject to dissolution, how should it be said that it should not depart? That indeed, is not possible."

"Therefore, bhikkhus, be ye an island unto yourselves, a refuge unto yourselves, seeking no external refuge; with the Teaching as your island, the Teaching your refuge, seeking no other refuge."

Having any confusion about what is proper and what is not, we can approach the question by investigation.

Even if we don't find the answer for a long time, our skills of investigation would probably grow; so this would help not only in solving that question, but also for all the future questions.

Feelings are the primary data for our mind. Therefore, in order to reach clean perception, it might be natural to accept feelings exactly as they come, not trying to change them in forceful ways.

Therefore, whatever feelings come, we can practice by

1. accepting them as they are,
2. observing whatever phenomena accompany them,
3. and investigating causes and results.

How can we investigate causes and results? For example, we observe how feelings are being created, and what phenomena they lead to.

This way we learn how various mental factors interact and develop. In that sense, we understand their meanings. This understanding of meanings constitutes the knowledge of what is proper and why exactly.

This practice will not only solve the problem, but also will give some insight in mechanisms working in our mind. Knowing those mechanisms, we could improve our practice, as well as help others better.

Let it be helpful!