[exim] Start working with exim config files

Hello,
I started working with Exim a few days ago, so I'm a very very beginner
for now. The question I have is, I think, very easy and the goal of my
question is to get a proper use of Exim's config files.
The setup :
I'm using a Debian distribution, so after installing Exim4 I done a :
dpkg-reconfigure exim4-config to set the main parameters of my network.
After that, I wanted to enable TLS and here is the problem...
I added the "MAIN_TLS_ENABLE = yes" in the
conf.d/main/03_exim4-config_tlsoptions. Here it works after restarting
Exim : I can see STARTTLS after EHLO localhost on telnet.
But after that I wanted to set other options like (for examples) :
tls_certificate = /etc/ssl/certs/file1.crt
tls_privatekey = /etc/ssl/certs/private/file2.key

(I know that Exim takes by default the exim.key and exim.crt in
/etc/exim4 folder so I can unset the two lines above and use the files
generated by gencert command... but it's to understand the good way to
set/redefine the parameters in config files).
So... I tried to insert theses two lines in ?
conf.d/main/03_exim4-config_tlsoptions ? and also after in a new file ?
conf.d/main/000_local ?... but Exim tells : "tls_certificate" option
set for the second time".
So I took a look to the files in conf.d and saw that this ?
tls_certificate ? parameters already exists like here :
.ifdef MAIN_TLS_CERTKEY
tls_certificate = MAIN_TLS_CERTKEY
.else
.ifndef MAIN_TLS_CERTIFICATE
MAIN_TLS_CERTIFICATE = CONFDIR/exim.crt
.endif

(If the MAIN_TLS_CERTKEY is defined, Exim takes this parameter. Else if
not defined, Exim use the default files like "/etc/exim4/exim.crt"
where CONFDIR is defined as /etc/exim4).
Here is my question : what is the best way to define a value like this
?
Create in conf.d/main directory a file called like ? 000_local ? and
set in ? MAIN_TLS_CERTKEY = value ? ? I prefer asking to advanced
admins what is the good way.

So with a second example, if I want to enable the ?
tls_try_verify_hosts ? value, I have to set :
MAIN_TLS_TRY_VERIFY_HOSTS='true' in the 000_local file in conf.d/main/
?
So with this each time you want to set a value and if it's already
defined in default conf, you have to find the variable associed ? Like
here, we wanted to set ? tls_try_verify_hosts ? and the associated
variable is ? MAIN_TLS_CERTKEY ?.
I know it's a really beginner / noob noob question... but just want to
go to the good way with Exim.
Thanks a lot.
Regards,

JohnS