The most basic calculation for a company is calculated by year revenue *10. Your company would be worth $40,000*12*10=$4.8M. The first round is usually around 20%, so you could raise $1M. This would make sense and investors would find your numbers convincing.
However, there are obviously also other factors at play. For instance, if you have gained your $40k revenue/month within a year or less, you can point to very fast adoption, so your valuation should be $10M. Then, you can argue that you need a lot of money for expansion in the U.S. and you want to go big and beat your competitors (which you hopefully have) to the market, so that you can seize the $8B global market first.
Then you can argue, that within the next 3 years, you anticipate $40M annual revenue, that's why you want to raise $10M at a $80M valuation.
Ok, now forget about all of these numbers. Understand that investors are led by emotion. They don't understand your business or cannot asses your predictions. Investors aren't engineers. They studied MBAs, never built a business before, then started as an Investment Analyst at a VC firm, after 5 years became partner and after another 5 years became limited partner. However, they never built a business and they couldn't for the life of it if you told them to.
So, you can simply tell them your great ambitions and they will be very excited and tell their boss about it. However, don't be cocky, overconfident or delusional, just be normally confident. The investor must think that he is more knowledgeable than you, more confident. Make it their idea (https://uniquedevelopment.com/blog/how-to-persuade-people-to-do-what-you-want-and-think-its-their-idea/).
They will forget about your current $500k revenue and only think about the $40M revenue within 3 years, on which you can raise $10M at a $80M valuation. This is feasible if you raise in the U.S.
However, if you raise in Europe, you can practically half your valuation if you want to raise within 3 months. In Europe, I suggest to raise $5M at a $30M valuation currently even and you should also tone down you ambitions, such that you estimate $20M revenue within 3 year.
I hope this helped.