Trade altcoins with lower buy limits?

Hello all!
I am new in the Crypto world and have a fundamental question regarding trading, where I could not find an answer in internet at all. (by the way sorry for my bad English).

Let me devide my problem in the two questions below.
I am not sure about, how to watch on a crypto price. Example: Stellar Lumen (XLM).

1) When I follow and analyse a chart, should I watch on the XLM/ETH (or XLM/BTC) chart, or should I watch on the XLM/USD chart?? Background of my question is the following: I can only buy XLM with ETH (or BTC), because it is like that on an exchange platform, right? (no I do not want to sign on XLM platform directly and buy there for USD).

Therefore, I am "forced" to follow the XLM/ETH chart (I trade in ETH not in BTC), at least to know, where I bought, and where I will (want to) sell.
But the XLM/ETH chart is completely different compared to XLM/USD chart. E.g. in ETH Stellar Lumen came back nearly to the bottom (0,00035). This is a minus of about 88% from top (0,00117)! BUT, in USD Stellar lost only 55%.

I think the effect is because ETH at the same time won a lot, but USD is stable. Am I right?

How (where) do you follow your altcoin? Altcoin/ETH(BTC) chart or Altcoin/USD chart?

2) I would like to set a low buy limit (e.g. again for XLM). As mentioned I have to trade (to buy) with ETH. Lets say, the current XLM/ETH price is 100 and i put a buy limit on 50. Now lets say, after 1 week of waiting XLM/ETH price comes down and I buy for 50. BUT when XLM/ETH price comes down, also (mostly) other altcoins and sometimes (after a bearish crypto time) also ETH comes down. Then during my 1 week of waiting time also ETH lost in value. BUT I was waiting for the XLM/ETH price of 50 in ETH!
So here I do not understand, which sense does it make to set a low buy limit when ETH also comes down. I would like to "park" my money in USD and then set a buy limit and buy XLM for USD (and not for ETH), because ETH also comes down together with XLM. Then mayby i catched the 50 limit, but i have less ETH to buy XLM (ETH came also down).

So my question is, how do you solve this problem? Does it make sence to set up a low buy limit in ETH (or BTC)?

Think I did not understand something important. But by now, did not figure out what that could be :-)
Would appriciate a lot a helping answer!