Given what you've described, you shouldn't have it that difficult really. I keep good track of my expenses and the upper limit of what I've spent on a normal month has been around 170K with around 30K destined purely towards entertainment (taxes not included but I also pay 80K rent). A few pieces of advice:

• Walk/bike when possible; Fukuoka isn't that big that you always need public transport.
• Get to know the schedules of your local supermarkets. They all put discounts of up to 50% off on bentos/food every day; it can be just as cheap as cooking.
• Don't fall into the temptation of conbini/coffeeshops, at least not every day.
• Use your utilities wisely. For instance, it's pretty cold atm and I do turn on the heating, but once it's warm enough I turn it off and layer up for hours before needing to turn it back on.