How much household savings to target annually?

[Background]

So I and my wife both (38) will be working in Japan very soon and our household expenses would be in the range of 6.2M to 7.3M annually, thanks to the international school fees for my 6 year old and high rental costs in Yokohama.

We plan to live for long say at least 15+ years in Japan, and possibly even retire here. (We had already been here previously for 7 years, went back to our home country and are now returning back).

[Question]

I want to know how much savings should I target? Is annual 2.4M savings good enough? That would be ~20% of in hand household income. I read on Japanese websites that average Japanese person's savings is pretty low, and an ideal range is 20~30% of annual income. Ideally I would like to save 3.6M JPY (~30%) annually. What is considered a good % savings w.r.t. to annual in hand household income?

[Additional background. Ignore if not interested]

Depending on this number I need to find out how much should I negotiate on the salary. I am currently in final round interviews with 4 companies and one other has already expressed their intention to share an offer letter.

My wife would be getting around JPY 550k-600k in hand per month (thats around 6.6M in hand annually), as her job/salary is already decided.

But mine is not and I need to adjust the negotiations as per the target savings amount.

This savings amount will be used for creating an emergency fund first and then investments in Japan itself (for possibly buying a house in the future and/or for retirement, and kids higher education). Note – We already have liquid assets more than JPY 30M in my home country but I want to keep them there for now, unless absolutely necessary. I want to build Japan assets using Japan income only as much as possible.

Please help me.

by shizukadane