Interpreting an Order Book (example Kraken.com)

An (forex) Order Book (OB) for a trading (forex) pair (e.g. XBT-USD) has ASK and BID rows. Each row has PRICE and VOLUME (at least). Each row represents an offer for selling or buying a maximum of VOLUME amount at PRICE.

Starting with a START-AMOUNT in BASE currency, how can I calculate exactly how many rows, starting from the 1st, i.e. the "cheaper-for-me"?

It is a simple calculation yet I am confused:

1. as to whether I need to use the ASK or BID rows.
2. as to what currency is VOLUME in.
3. as to whether VOLUME in ASK and VOLUME in BID are in different currencies.
4. Sometimes I will need to buy USD with XBT and so I will need the USD-XBT OB which does not exist (at least in kraken.com where I focus), can I use XBT-USD OB in order to buy XBT with USD? How?

Additionally, how should I use the "pair decimals" and "lot decimals" for the specific trading pair in order to do accurate, integer calculations? Note: terminology specifically refers to kraken.com's OB from the kraken.com's Asset-pair metadata but mainly I am asking: if someone tells us that in a specific book, currency (I guess BASE?) is expressed in 1 decimals and volume in 8, how can I do the calculations in integers to be accurate. (I realise that can be a question on its own and can ask it separately if people suggest it).

I am not sure if different forex platforms have different conventions, if they do then I am working with kraken.com . Here is some example data specific to kraken.com:

ASKS:
45867.70000,0.002
45870.90000,0.001
45873.80000,0.001
45875.90000,0.379
45880.70000,0.215
BIDS:
45867.60000,3.708
45867.30000,0.186
45866.80000,1.090
45865.90000,0.436
45865.50000,0.030


XBTUSD metadata (get it with this API call from browser):

...
pair_decimals:1,lot_decimals:8
...


p.s. I have very little experience with finance, more with programming and algorithms.