UK Visit Visas Refused Due to Evaluation Oversights and Similar Errors

To get to the bottom of this we have to start with what they mean by "What is your total monthly income from all sources of employment or occupation after tax?" For most people it means what's in their paycheck after taxes. But for a few others, it's more complex, especially if they get income from multiple sources, and there's an extra layer of complexity if they get income denominated in multiple currencies. The form needs to accommodate both of those types of cases.

What the applicant is meant to do is to roll up their world-wide income and convert to their base currency. Here's a screen cap of the application where someone is reporting a world-wide monthly income of 18,500 Egyptian Pounds...

At the applicant's discretion, they can also report the equivalent amount in Sterling. Here's the associated screen cap...

Note that each time the entry is denominated using the appropriate ISO code. This is what ECO's want to see and if the applicant is vague the ECO will interpret the figure as the applicant's base currency. You were vague, so that explains the ECO's assumptions (which led to your disappointment).

Expenses, on the other hand, are a bit different. Expenses are reported in Sterling because if a person is visiting the UK they will be spending Sterling and it gives all applications a common reference point. So to keep things consistent, all expenses are reported in Sterling. Here's the relevant screen cap...

Exchange rates in a visitor application are meant to be reasonable approximations and they will give a reasonable amount of leeway. So while it is not required, an applicant using "best practices" has the option of adding something like...

Doing this helps to convey that the applicant is treating the application with diligence and due sobriety.

Finally, people are advised to read the guidance. The guidance contains instructions for each element of the form, and for your question it says...

The part I have highlighted in red says...

Please state your total monthly income after taxes have been deducted. Please state which currency e.g. US Dollars or UK Pounds Sterling.

To summarize: when completing a UK visitor application, don't be vague. This was a fair cop.

For your other questions about using their complaints procedure, you can always complain if it helps get things off your chest. Based upon the refusal notice however, they are not going to change their decision or even offer you a gratis application next time.