The eighth precept refers to using high and luxorious beds and seats. Suppose one was at a café or other public place using a laptop to study Dharma. Would it be a violation of the precept to use a basic chair and table? Sitting on the floor wouldn't seem to be socially acceptable in these situations.
For practicality reasons, some of the rules are not applicable when navigating through modern day society. Even some monastic rules can be quite absurd but one has to be self-reflective enough to determine that for themselves. When you find yourself in luxurious surroundings, it's an opportunity to examine how your mind is responding to that type of environment.
The opportunity to practice is always available in any situation.
When using a laptop... sitting in a cafe... fearing sozial lose..., possible using others wlan, "enjoying allowed tonics" and others resources... a question, "is it suitable to sit on a chair?" is really pointless, isn't it? That is how the Uposatha observance pulls one out of dangers, by simple precepts. (and to get it not wrong, yes, it would be merely crazy to play the occupy game and sit on the flour while not having left the benefits of common society). Brahmacariya actually "cuts off" of socializing as usual, but to tend toward societies living differently. Maybe this is of help for getting the meaning of purpose, step by step: Methuna Sutta: The Discourse on Coupling (The seven bonds of sexuality).
It's not so, like in other answer suggested that Dhamma should be modified that it fits to ones usual ways but to pull one willing outof Samsara on the lane of letting go and liberation.
Sangama [fr. saŋ+gam] 1. meeting, intercourse, association Sn 681; J ii.42; iii.488; v.483. -- 2. sexual intercourse M i.407; J iv.106.
Talk in German: Sozialisieren - សង្គម [Sangama]
If willing to come down from a householders-chair, there are some explainings on how to observe preceptsn Rituals & Ceremonies - [Rituale & Zeremonien] sikkhāpada & pūjāvidhi
(It's possible of use to chance posadha into Uposatha in the question header)
[Note: this is a gift of Dhamma, not thought for trade, stakes, exchanges or other gains subject toward decay and should be deleted if it's not giften to give in Dhammic conditions]