Here are the short answers to your various questions –
If you want to be mathematically correct in coding your EA, then use these rules:
(1) If you are trading the EUR/GBP pair, use the GBP/USD BID price.
(2) If you are trading either the EUR/AUD pair or GBP/AUD pair, use the AUD/USD BID price.
(3) If you are trading EUR/NZD, GBP/NZD or AUD/NZD, use the NZD/USD BID price.
(4) For every other traded pair XXX/YYY, use the USD/YYY ASK price.
For a detailed explanation of these rules, see the “long answer” below.
Maybe it doesn’t matter, at all, depending on how much precision you demand in the calculated result.
In your post, you stated, “10CAD/1.24900 = 8USD”. That’s an approximation.
Actually, 10 CAD / 1.2490 = 8.006405 USD. And 10 CAD / 1.2500 = 8 USD.
If 10 pips, one way or the other, in the USD/CAD exchange rate doesn’t matter, for your purposes, then don’t bother with (1), (2), and (3) above, and just use the ASK price for every conversion.
No. The pip-value doesn’t depend on whether you are LONG or SHORT, or whether the price increases or decreases.
Okay, here’s the long answer to your first question –
Trading pairs of the form XXX/YYY involves pip-values of 10 YYY per pip, per standard lot traded.
For an account denominated in USD, the 10 YYY pip-value must be converted into USD.
The conversion into USD will depend on whether price information is quoted in terms of USD/YYY or YYY/USD.
There are only four pairs in which the USD is the quote currency: EUR/USD, GBP/USD, AUD/USD, and NZD/USD.
• EUR will never be the YYY currency in any pair you trade, because EUR is always the base currency in every EUR pair.
• As for GBP, there is only one pair in which GBP can be the YYY currency (the quote currency), and that is the EUR/GBP pair.
• As for AUD, there are only two pairs in which AUD can be the YYY currency (the quote currency), and they are EUR/AUD and GBP/AUD.
• And finally, as for NZD, there are three pairs in which NZD can be the YYY currency (the quote currency): EUR/NZD, GBP/NZD, and AUD/NZD.
So, for the following six traded pairs, EUR/GBP, EUR/AUD, GBP/AUD, EUR/NZD, GBP/NZD, and AUD/NZD, the pip-value conversion to USD involves the YYY/USD pair. That is, the conversion involves either GBP/USD, AUD/USD, or NZD/USD.
For all other traded pairs, the pip-value conversion to USD involves the USD/YYY pair. Examples: USD/CAD, USD/CHF, etc., and even USD/JPY (although, as you have noted, the original pip-value calcuation for yen-pairs is different from all the other currency pairs).
So, to summarize (as stated above in the “short answer”) –
(1) For traded pair EUR/GBP, use the GBP/USD BID price, as if you were selling GBP and buying USD.
(2) For traded pairs EUR/AUD and GBP/AUD, use the AUD/USD BID price, as if you were selling AUD and buying USD.
(3) For traded pairs EUR/NZD, GBP/NZD, and AUD/NZD, use the NZD/USD BID price, as if you were selling NZD and buying USD.
(4) For every other traded pair XXX/YYY, use the USD/YYY ASK price, as if you were buying USD and selling YYY.