I choose my bases by the bounce they recieve.. ETC bounced from 81ish to 96.. the 78 you are refering to had no bounce (on the Bittrex chart) so it doesnt qualify as an area where buyers were willing to move in on the price and buy...
As far as your question on coin pairings.. Everything relates to eachother in the market.. So lets say ETC is paired with ETH... but ETC is also paired with BTC.. if BTC stays constant but ETH falls in value on the ETH/BTC pair, then what happens to ETC/ETH pair.. because all three are paired against each other..But its only ETH thats falling.. so should ETC lose BTC value? Or should ETC/BTC stay the same but ETC/ETH go up, since ETH is going down.. its pretty complicated to think about, but each coin in a pair effects the other pair relationships.. So basically if one coin is having a big move, in relation to BTC then you can expect other coins to react that are connected in pairing.. And i know that doesnt give you an exact answer but there really is no metric for this, because its not like the stock market, where there is one base currency, its like forex, where pairs are effected by other pairs.. Thats why its always best to trade the chart, and not let things get too overly complicated.