cell phone repeater: DB Gain?

My wired connection from a Yagi antenna to Pantec Modem yields anywhere from 81dB to 91dB (Verizon signal), with an average of, say, 86 dB. If the dB scale stops at 50, is the "65dB Gain" signal amplifier I am looking to buy a waste of money and amp power? Looks like I can only 'gain' 35dB or so....do I buy a 35dB gain amp?

Also, suppose I install an amp and interior antenna, and as a result I have boosted signal strength broadcast throughout my house. My Pantec Modem, 10-12 feet away from the antenna, picks up the boosted signal...so my desktop internet connection is better than before. But then we buy a tablet....and use the same provider - Verizon. Can you quantify/estimate what type of slow down we might experience by using two devices at once through this repeater set up ? Do I use a different provider for the tablet for best speed? Thanks for your help!