How can I calculate my anticipated peak current load?
To answer the actual question here :

The maximum sub-fuse is 40A. So upgrading to 40A seems logical.

No, the 40A device you're looking at is probably this one :

This is a GFI/RCD trip device and its main purpose is to provide ground fault protection for the dwelling.  It does have a 40A rating, but this does not impact your user load calculations in any way.  What you need to consider is only the applicances you will be using, and the schedule that you will be using them on.
As an aside, the service breaker here is the green one on the left - this is the breaker TEPCO will change if you upgrade your service.  I suspect if you ask for a service upgrade to something greater than 40A they will also replace the ground fault unit to match the service rating.

Would I be correct in doing the following?
calculate maximum amperage with voltage and wattage of each device
summing up the amperage of all of the devices (ie. 10A + 20A + 7A + 5A)
finding thus the maximum amperage of the main fuse

You can do this if you anticipate that you will be using all of your electrical devices at the same time.  If, however, you are trying to make a better guess of your peak load, you should consider all your appliances and figure which combinations of them you will likely be using at the same time, and then select the highest load use case you expect you will encounter.
For example, if you never plan to use the microwave while drying your hair, you don't need to consider the load of both of those appliances at once.  If you're really trying to save on a rate plan, you can also then develop a daily routine that deliberately places certain activities at different times to reduce your peak electrical usage.  If you're fine turning off the A/C, for example, while you use other high-load devices for a short activity, this can really cut down on the service size (and cost) for your electricity in Japan.
This kind of mindful load balancing is very much encouraged in Japan, and the steep rate pricing for larger service is very much designed to promote this kind of peak-shaving behaviour.  It reduces the peak grid load if everyone is mindful to not dry their clothes and make rice at the same time, so this kind of lifestyle hacking is very much the norm in Japan, especially since the setsuden push following the loss of Fukushima.
There is a good article at Japan Times that discusses these types of strategies.

Service Types
Also, keep in mind that there are two types of common electrical service in Japan.  The panelboard in the image above (which we now know looks &quot;exactly&quot; like yours) is showing a two-pole service feed, or what is called in Japan a &quot;single three&quot; (単3) service.  &quot;Three&quot; here refers to the number of wires - two hot and one neutral.  A 単3 service at 30A provides two hot legs at 100V, so 6kW of total power, or effectively 60A of 100V power.  This type of service also allows for 200V loads for larger appliances, A/C, etc (note the 200V breaker top-right above, for example).  200V loads draw from both legs, so they must be budgeted separately.
The other common type of service is a &quot;single two&quot; (単2) service, or single pole, two wire service.  This type of service provides only one hot leg and 100V power only.  A 30A 単2 service can only provide 3kW, not 6kW like the 30A 単3 service, so when calculating how large your service amperage needs to be you need to be sure which type of service you have.  Harper's excellent answer below provides more detail on how to calculate your loads for both situations.