yes, people in service, like the security guard at your building, or even when you're parking around town. helpers, maybe waiters (ONLY familiar ones of restaurants where your family frequents). sometimes in a mean way, we think of it as kind of a 'bribe' for service the rest of the year.. haha.. anyways, but it's also in way for wishing good health and good luck for the new year. so we're all happy giving and receiving, it's really like handing out a wish to people around you.
about how much you pay.. it's not really a percentage, doesn't really have to be whole month of salary, that is qutie a lot. basically, it is just for the gesture, so for service people, very standard are HKD20, for those who are closer HKD100, and e.g. my MIL gives head waiters of the restaurant we always goes to HKD500. and we get great service

e.g. we give our helpers HKD100 which is a small percentage of her monthly wage, but they are still thrilled about it. it really is much less about how much in lai see.
i always see them as 'lai see' and not money, the meaning of 'lai see' means everything goes well, good luck, prosperity. so in spirit is not really how much it's inside.
some sayings:
Kung Hei Fat Choy (very common, wishing for prosperity)
Man See Sing Yee (Man See = ten thousand things, Sing Yee=goes as you wish, wishing that every goes according to your wishes)
Sun Tae Kin Hong (Sun Tae = body, Kin Hong = health, wishing you good health)
Lung Ma Ching Shun (Lung = dragon, Ma = horse, Ching Shun = feeling bright, wishing you good spririts and health like a dragon and horse)
Chan Dou D (Chan is make, Dou D is more, basically means wish you makes more money, very useful in a place like HK)
Bo Bo Ko Shing (Bo Bo = steps, Ko Shing = going up, wishing you climb up the ranks, for people who are working)
Hok Yip Chun Bo = (Hok Yip = education/studies, Chun Bo=improve, wish you improve on your studies, for students)
Fa Ko Cheung Dai = (Fa Ko=fast tall, Cheung Dai = grow big, wishing you grow up fast

sounds weird in English, but for kids)
have lots more but will stop here. i had fun typing them out.. feel free to ask more!
Sing Dan Fa Lok (Merry Christmas)